Nanomaterials‐Induced Redox Imbalance: Challenged and Opportunities for Nanomaterials in Cancer Therapy

Abstract Cancer cells typically display redox imbalance compared with normal cells due to increased metabolic rate, accumulated mitochondrial dysfunction, elevated cell signaling, and accelerated peroxisomal activities. This redox imbalance may regulate gene expression, alter protein stability, and modulate existing cellular programs, resulting in inefficient treatment modalities. Therapeutic strategies targeting intra‐ or extracellular redox states of cancer cells at varying state of progression may trigger programmed cell death if exceeded a certain threshold, enabling therapeutic selectivity and overcoming cancer resistance to radiotherapy and chemotherapy. Nanotechnology provides new opportunities for modulating redox state in cancer cells due to their excellent designability and high reactivity. Various nanomaterials are widely researched to enhance highly reactive substances (free radicals) production, disrupt the endogenous antioxidant defense systems, or both. Here, the physiological features of redox imbalance in cancer cells are described and the challenges in modulating redox state in cancer cells are illustrated. Then, nanomaterials that regulate redox imbalance are classified and elaborated upon based on their ability to target redox regulations. Finally, the future perspectives in this field are proposed. It is hoped this review provides guidance for the design of nanomaterials‐based approaches involving modulating intra‐ or extracellular redox states for cancer therapy, especially for cancers resistant to radiotherapy or chemotherapy, etc.


Introduction
Cancer remains a leading cause of death worldwide. [1]Despite the advancements in cancer treatment modalities, the genetic heterogeneity, diversity, and complexity of tumors pose daunting challenges for developing more efficient and effective cancer therapies. [2]Abundant data revealed that cancer cells experience oxidative stress during certain critical phases of their evolutionary and progressive stages. [3]echanisms leading to oxidative stress in cancer cells may involve hyperactivation of anabolic pathways, [4] increased mitochondrial function, [5] malfunction of the electron transport chain (ETC) as a result of mitochondrial DNA mutations, [6] and oncogenic pathway activation. [7]Consequently, cancer cells exhibit redox imbalance compared to normal cells and deploy a variety of antioxidant mechanisms to counterbalance the oxidative damage resulting from oxidative stress.These mechanisms include glutathione (GSH), [8] thioredoxin (Trx, aka TXN), [9] antioxidant enzymes (e.g., glutathione peroxidases (GPXs), [10] catalases (CATs), [11] and superoxide dismutases (SODs), [12] and their transcriptional regulators, such as the nuclear factor erythroid 2-related factor 2 (Nrf-2) [13] and BTB domain and CNC homolog 1 (BACH1). [14]herefore, employing redox-modulating strategies to specifically target these distinctive biochemical attributes of cancer cells presents a feasible therapeutic approach for cancer treatment.Indeed, various cancer treatment methods involving the regulation of redox levels have been widely reported in recent years.They are basically divided into three main categories: 1) enhancing the metabolic capacity of the antioxidants and antioxidation enzymes to suppress the tumor growth; 2) inducing excess highly reactive substances in cancerous cells to exceed the safe threshold and leading to the demise of these malignant cells; 3) disabling the cellular antioxidation systems in the tumor itself to prevent the clearance of reactive substances in cells. [15]However, it has been frequently observed in clinical trials that employing antioxidants as cancer therapies can actually stimulate the emergence of additional phenotypes in malignant tumors, and tend to increase cancer incidence and cancer-related deaths. [16]Many natural or synthetic ROS-inducers have been found to promote the generation of ROS by affecting the ETC, redox cycling compounds, and disrupting the antioxidative defense mechanisms.Most chemotherapeutic drugs, such as doxorubicin (Dox), cisplatin, 5-fluorouracil (5-FU), and arsenic trioxide (ATO), can kill cancer cells by directly or indirectly facilitating ROS accumulation. [17]onetheless, the broader application of these agents in cancer therapy is substantially hindered by their lack of specificity and the associated systemic toxicity akin to the limitations encountered with traditional chemotherapy and radiotherapy regimens. [18]he ability of nanomaterials to respond actively to local microenvironments and enable precise spatial and temporal functions confers diverse advantages for their utilization in cancer therapy.A series of stimuli-responsive nanomaterials has been meticulously engineered and applied for the treatment of cancer.These materials are designed to either directly boost the production of ROS or disrupt antioxidative defense mechanisms, potentially resulting in cell death via apoptosis, autophagy, ferroptosis, or necrosis. [19]Given this rapidly increasing interest in using nanomaterials for modulating redox imbalance in cancer cells, it is imperative to provide a comprehensive overview of the diverse stimuli-responsive nanoplatforms available for regulating redox equilibrium within cancer cells.While numerous excellent reviews have described the realm of nanomaterials pertaining to oxidative stress, [20] there is a lack of reports addressing the design of nanomaterials tailored to enhance ROS/reactive nitrogen species (RNS) production, disrupt endogenous antioxidant defense systems, or manage reduction imbalances, either individually or in combination.Here, we first delineate the physiological features of redox imbalance in cancer cells and highlight the challenges in modulating redox state in cancer cells.Then, we systematically categorize and expound upon nanomaterials designed to regulate redox imbalance according to their ability to target redox adaptations in cancer cells, particularly focusing on stimuli-responsive nanomaterials.Finally, we offer insights into the future prospects of this field.We hope this review provides comprehensive information on different methodologies that bear an outstanding potential to further promote this very promising field from basic theory to clinical application.
Unlike normal cells, hyperproliferative cancer cells have greater demands for the energy required to promote macromolecular biosynthesis and the generation of the essential precursors of amino acids, nucleotides, and lipids. [25]Moreover, cancer cells must undergo metabolic rewiring to contend with the often-decreased availability of nutrients, increased acidity, and onset of hypoxic conditions prevalent in the densely populated and expanding tumor microenvironment (TME). [26]In response to these demands, cancer cells undergo metabolic reprogramming, leading to alterations in antioxidant-to-oxidant homeostasis.Although a comprehensive assessment of the redox status across various cell types and different phases is currently lacking, [27] most cancer cells exhibit elevated levels of ROS [28] /RNS [29] in the early phase of mitophagy.For example, when compared to normal tissue, where the H 2 O 2 levels typically range ≈0.5 × 10 −3 μmol 10 −4 cells, [30] the concentration of H 2 O 2 in tumors can be as high as 100 μm. [31]To evade senescence, apoptosis, necrosis, and ferroptosis, cancer cells strategically modulate multiple antioxidant defense mechanisms for the proper elimination of increased levels of ROS/RNS to establish new redox homeostasis. [32]For example, increased expression or activity of antioxidative-related enzymes such as SOD and its mimics, CATs, GPXs, and Prx have been demonstrated in many different cancers.The level of thiol-based antioxidants GSH, Trx, and Prx is also upregulated to scavenge the excessive ROS. [9,33]In addition, the appropriate levels of ROS in the short-to-medium time span will trigger the upregulation of antioxidant transcription factors, such as HIF-1 or Nrf-2, to support cancer cell proliferation by genetic reprogramming [33][34] (Figure 1).However, redox alterations in cancer cells are very complex.For example, cancer stem cells and some drug-resistant cancer cells maintain ROS levels to low amounts, which is mainly due to the expression of ROS scavenging molecules and their efficient DNA repair systems, thereby promoting resistance to oxidative targeted therapies.Cancer cells maintain this "tense and fragile balance" by counteracting the elevated ROS level with the alteration of redox signaling pathways responsible for increased antioxidant synthesis to promote the rapid proliferation, invasiveness, metastasis, and other malignant phenotypes of tumor cells. [35]For more extensive exploration of redox homeostasis features, readers are referred to other comprehensive reviews. [27,34,36]

Nanomaterials that Manipulating Redox Balance in Cancer Cells for Cancer Therapy
As outlined in the preceding sections, the dysregulated redox systems within cancer cells significantly contribute to processes like proliferation, survival, and metastasis in various cancer types.Consequently, perturbing the delicate tumor redox balance within tumors serves as a promising anti-tumor approach with tumor selectivity.Therapeutic approaches either promoting reactive oxidative substances production or disrupting the antioxidant defense mechanisms in cancer cells could result in a dramatic increase in the generation of reactive oxidative substances to the point where they trigger cell death.Various drugs that manipulate redox equilibrium have achieved promising outcomes in killing cancer cells, however, the adverse effects and therapy resistance induced by reactive oxidative substances limit their therapeutic potential.The designable, multi-functional, and modifiable properties of nanomaterials provide many opportunities to manipulate redox imbalance in cancer cells for cancer therapy and enhance the antitumor effects of pro-oxidants.In the past few years, numerous responsive nanomaterials could manipulate redox imbalance in cancer cells for cancer therapy through three kinds of action mechanisms: promoting the generation of highly reactive substances, disrupting the antioxidative defense mechanisms, or increasing the production of both.In this section, we summarized the nanomaterials that manipulate redox balance in cancer cells for therapeutic purposes, with a particular focus on stimuli-responsive nanomaterials.Based on these mechanisms of action, these responsive nanomaterials are classified into three main categories: 1) inducing excessive reactive oxidative substances under internal stimuli or external stimuli beyond the toxic threshold to induce cell death and tumor destruction; 2) elevating reducing agents in cells to disturb redox balance; 3) combining the inhibition of the antioxidant mechanisms with boosting ROS/RNS production (Figure 2).

Responsive Nanomaterials That Induce Excessive Reactive Oxidative Substances
Living species are continuously subjected to more than 20 kinds of chemically reactive species containing ROS, RNS, and their secondary products that are produced by internal sources (mitochondria, xanthine oxidase, phagocytes, reactions involving iron and other transition metals, peroxisomes, arachidonate pathways, exercise, ischemia/reperfusion, inflammation), external sources (environmental pollutants, tobacco smoke, radiations, certain drugs, etc.) and physiological factors (mental status and disease conditions). [37]Examples of ROS, RNS, and their secondary products include hydroxyl radical (•OH), hydrogen peroxide (H 2 O 2 ), superoxide (O 2 • − ), peroxyl (ROO•), and alkoxyl radicals (RO•), hypochlorous acid (HClO), organic peroxides (ROOH), aldehydes (HCOR), nitric oxide (NO•), nitrous oxide (N 2 O), peroxynitrite (ONOO − ) and its protonated form (ONOOH), nitrogen dioxide (•NO 2 ), nitrosothiols, etc. [22] Below a certain reactive oxidative species threshold, these molecules play a crucial role in the regulation of several cellular processes such as cell signaling, proliferation, differentiation, and cell death.However, the unpaired electrons of reactive oxidative species allow them to grab electrons from the surrounding substances indiscriminately and induce non-specific damage to proteins, lipids, and DNA, resulting in senescence, degeneration, or fatal lesions in cells. [38]Therefore, above the reactive oxidative species threshold, reactive oxidative species can trigger apoptotic signals. [39]It is now known that many metabolically transformed and fast-growing cancer cells have higher reactive oxidative species levels than neighboring normal cells.This provides a possible therapeutic window to eradicate cancer cells rather than normal cells by stimulating endogenous reactive oxidative species generation in tumor cells.Various nanomaterials and strategies have been explored for inducing excessive reactive oxidative species generation beyond the toxic threshold to induce cell death and tumor destruction.They are basically involved in three main categories: 1) inducing the generation of highly reactive oxidative species directly in tumor cells under internal stimuli, such as H 2 O 2 and glucose; 2) inducing the generation of highly reactive oxidative species by external stimuli, such as light, ultrasound (US), X-ray irradiation, etc.; 3) inducing the generation of highly reactive oxidative species through regulating endogenous oxidation signaling pathways (Figure 3).

TME-Responsive Nanomaterials That Generating Reactive Oxidative Species Under Internal Stimuli
The abnormal metabolism and metabolite accumulation in cancer cells lead to the development of a unique TME during their evolution.The TME typically exhibits a low pH, high GSH concentration, excessive H 2 O 2 , severe hypoxia, etc. [40] These characteristic features provide numerous potential targets for the development of internal stimuli-responsive nanomaterials designed to induce excessive reactive oxidative species.These reactive ox-idative species encompass ROS (•OH, 1 O 2 , •O 2 − ), RNS (NO, ONOO − ), and other reactive oxidative species (carbon-centered radicals like CO and alkyl radicals, sulfate radicals).Table 1 summarizes the redox capacity, half-life, pH tolerance, selectivity, chemical stability, and antitumor mechanism of these active species used for cancer therapy.
Generation of •OH: •OH, a highly reactive ROS, has a very high oxidation potential (2.7 eV) among radicals (Table 1).The prevailing design principle for generating •OH involves Fenton or Fenton-like reactions, typically based on transition metal elements with multiple valences. [41]These transition metal elements with multiple valences play a significant role in TME manipulation.They facilitate the conversion of the overexpressed H 2 O 2 into •OH through an electron transfer process, thereby consuming the reducing GSH and ultimately achieving overall regulation of TME. [42]Based on transition metal-based Fenton/Fenton-like reactions and the TME, Shi's group synthesized amorphous iron nanoparticles (AFeNPs) and used them as theragnostic agents (defined as chemodynamic therapy: CDT). [43]onization of the AFeNPs enables on-demand ferrous ion release in mild acidity of the tumor and subsequently promotes the disproportionation of overproduced H 2 O 2 to produce •OH at the tumor site (Figure 4a).The lethal •OH is capable of impairing DNA and converting polyunsaturated fatty acids to lipid peroxides, followed by the activation of a series of antitumorigenic pathways, thus inducing cell apoptosis/ferroptosis to achieve tumor treatment.Considering that Fenton/Fenton-like reactions are difficult to trigger under alkaline conditions and that the None [ 88] Unstable [ 89] The high oxidative capacity of •OH causes chain oxidation reactions that lead to oxidation of the surrounding lipids, proteins, and DNA, thereby resulting in damage to the cell [ 90] 1 ≈10 −6 s [ 92] 3-11 [ 93] Higher affinity for unsaturated carbon or amine groups [ 94] Strong versatile inorganic ions resistance [ 94] 1 O 2 quickly oxidizes the sulfur-containing or aromatic amino acids in proteins and also causes strong damage to lipid membranes [ 95] •O 2 − 0.31 V [ 96]   ms-hours a) [ 97]   Wide [ 98] Attacking the positively charged components of any organic species in the absence of protons [ 98]   Unstable [ 98] •O ONOO, causing lipid peroxidation damage and mitochondrial dysfunction [ 99]   NO• 1.2 V [ 100]  0.09-2 s in vivo [ 101] Wide [ 102] Readily reacting with metal complexes, hypervalent complexes, and hemoglobin [ 102] Stable for biomolecules whose orbitals are completely filled [ 102] Lipid peroxidation and nitrosative stress triggered by NO• lead to DNA and plasma membrane damage, resulting in tumor apoptosis [ 103]   ONOO − 105b] Unstable [ 106] ONOO − mediates protein oxidation and nitration, lipid peroxidation, and mitochondrial dysfunction, ultimately leading to cancer cell death [ 107] NO 2 − 1.03 V [108]   110 s [ 109] -Rapidly reacting with organic compounds such as biological lipid membrane [ 110] or hemoglobin, [ 109] and exhibits moderate reactivity with phenoxide ions, anilines, phenothiazines, thiols, and ascorbate [ 108a] Unstable [ 111] NO 2 − induces an increase in intracellular calcium ions and ROS/RNS levels, leading to endoplasmic reticulum stress and mitochondrial dysfunction [ 112] •SO ≈3-4 × 10 −5 s [ 114] 2-9 [ 115] Selectively attacking specific chemical bonds [ 116] Unstable [ 117] The high oxidizing capacity and long diffusion distance of •SO 4 − can efficiently mediate oxidative stress in tumor [ 118] •Cl 2.47 V [119] --Rapidly reacting with electron-rich moieties [ 119b,120] Unstable [ 121] •Cl causes one-electron oxidation with DNA skeletons to inflict severe cellular damage directly [ 122] HOCl 1.48 V [123] 1 × 10 −7 s [ 124] 3-6 [ 125] Rapidly reacting with the sulfur-containing amino acid side-chains of Met and Cys [ 126] Relatively stable [ 127] HOCl selectively meditates malignant cells to apoptosis [ 128] ABTS• + 0.68 V [129]   8 × 10 −3 s 5-9 [ 130] Almost none [ 131] Stable in low temperatures (below 5 °C) [132] The thermal-labile azo initiator in the tumor can decompose under heating or US to generate two ABTS• + to achieve oxygen-independent tumor killing [ 133]   •CF Very short (<10 −10 s) [ 134] 4.8-9.6 [ 135] Reacting with 18 of the 20 common amino acids, which exhibit relative inertness toward •OH [ 136] Unstable [ 134] •CF 3 resulted in downregulation of ERCC1 in chemotherapy, which inhibit the repair of cisplatin-damaged DNA [ 137] N 3 • 1.32 V [138]   <2 × 10 −8 s [ 138] 4-13 c) Selectively oxidizing aromatic amino acid residues in proteins [ 138] Unstable [ 138] The generated N 3 • radical in tumor cells can destroy lysosomal membrane structure without oxygen [ 139] a) 138] Figure 4. Schematic illustration of TME-responsive nanomaterials that generate ROS under internal stimuli through Fenton or Fenton-like reactions for cancer treatment.a) The application of AFeNPs in cancer therapy by localized Fenton reaction.Reproduced with permission. [43]Copyright 2016, Wiley-VCH.b) Copper peroxide nanodots release Cu 2+ and H 2 O 2 under acid TME to generate •OH through a Fenton-like reaction, thereby achieving H 2 O 2 self-supplying CDT process.Reproduced with permission. [64]Copyright 2019, American Chemical Society.c) Chemodynamic and chemiluminescenct system based on ultrathin MnO x nanosheet and semiconducting polymer nanoparticles.Reproduced with permission. [77]Copyright 2020, Elsevier.d) Co single atom on N-doped porous carbon (Co-SAs@NC) as a bifunctional nanozyme triggered production of •O 2 − for synergistic tumor therapy.Reproduced with permission. [85]Copyright 2022, Wiley-VCH.
GOD-loaded magnetic Fe 5 C 2 core-shell structured nanosystem (Fe 5 C 2 -GOD@MnO 2 ) to provide a source of H 2 O 2 and O 2 .The acidic TME allows for the decomposition of the MnO 2 shell into Mn 2+ and O 2 , and can also trigger the release of GOD.The released GOD can effectively catalyze glucose to generate plenty of H 2 O 2 in the tumor region under sufficient oxygen conditions, thus accelerating the subsequent Fenton reaction catalyzed by the Fe 5 C 2 magnetic core in mildly acidic TME. [49]Moreover, it has been reported that the overexpressed ferritin heavy chain (FHC) in cancer cells can catalyze rapid oxidation of Fe 2+ into less reactive Fe 3+ and sequester Fe in ferritin, which significantly diminishes the antitumor efficacy of the Fenton reaction.47a] Generation of 1 O 2 : 1 O 2 represents the first excited state of the dioxygen molecule, displaying heightened reactivity, and is one of the ROS species generated during the oxidative phosphorylation (OXPHOS) process. [73]It readily reacts with biological molecules like unsaturated lipids, and  amino acids of proteins (tryptophan, histidine, methionine), which are integral components of cell and nuclear membranes.This interaction leads to the generation of a range of other reactive intermediates and disrupts the redox homeostasis of tumor cells. [74]Furthermore, it has also been clarified that the generation of 1 O 2 outside the cell membrane can selectively inactivate membraneassociated catalase, consequently re-activating intercellular ROS-RNS-driven apoptosis-inducing signaling. [75]Given its potential to achieve selective apoptosis or necrosis through localized generation, coupled with its mild redox capacity (2.2 V/NHE, NHE means normal hydrogen electrode), long half-life (1 μs vs ≈1 ns (•OH)), strong versatile inorganic ions resistance, and broad pH tolerance (Table 1), 1 O 2 treatment holds promises as a target therapeutic strategy.However, the most convenient and efficient way to generate this species is through the photosensitized excitation of ambient triplet oxygen.Only a few metal-based nanomaterials consisting of Mo [59,76] and Mn [77] active centers have the capability to generate 1 O 2 without the need for light irradiation.For example, Song et al. have developed a high-efficiency chemodynamic and chemiluminescenct system comprising ultrathin MnO x nanosheet and semiconducting polymer nanoparticles. [77]he ultrathin MnO x nanosheets catalyze the conversion of endogenous H 2 O 2 to 1 O 2 under the trigger of acidity, representing a novel chemodynamic process (Figure 4c).The 1 O 2 produced by MnO x can then substitute light and specifically excite thiophenebased semiconducting polymer nanoparticles to emit photons for near-infrared chemiluminescent imaging, which greatly amplifies the generation of 1 O 2 .Besides, Zhao et al. have proposed an energy-free and tumor-specific 1 O 2 therapy by intravenous delivery of ClO − -loaded ZIF and concurrent intraperitoneal administration of ascorbate. [78]Under the TME, ascorbate produced H 2 O 2 that could diffuse into the tumor cells where it would react with ClO − released from nanocarriers to stoichiometrically produce 1 O 2 .
Generation of •O 2 − : It has been well known that the NADPH oxidases (NOXs) 1-3 can be activated via the assembly of cytosolic subunits and produce •O 2 − . [79]Low concentration of the generated •O 2 − is quickly decomposed by the SODs in the body. [80]wever, excessive exogenous •O 2 − readily reacts with proteins, DNA, and lipids, leading to irreversible damage to cellular components and disrupting cell metabolism. [81]Therefore, various approaches have been devised to generate •O 2 − , including type I photoreactions based on benzo phenothiazine or Nile blue NIR cationic photosensitizers, [80,82] biochemical reactions, and molecular drugs. [83]Dai et al. have developed a ROS-promoting combination drug delivery platform via a biocompatible metalpolyphenol networks self-assembly process by encapsulating Dox and platinum prodrugs in nanoparticles. [84]The platinum drug and Dox can activate NOXs to generate •O 2 − .Cai et al. constructed the Co single-atom nanozyme on N-doped porous carbon via Co−N coordination, which displayed TME-responsive cascade enzymatic activities with both catalase-and oxidaselike properties. [85]The cascade enzymatic reactions enable the decomposition of cellular endogenous H 2 O 2 to produce •O 2 − (Figure 4d).By incorporation of Dox as the chemotherapeutic agent, the single-atom nanozyme-based platform achieved significantly enhanced antitumor effects.
Generation of RNS: RNS plays an important role in cell signaling, immunity, and tissue homeostasis. [141]Typical RNS, including NO, nitroso ion (NO + ), ONOO − , NO 2 , Snitrosomercaptan (SNOs), and nitrogen oxides (NO x ), are derived from the interaction between NO and various ROS [142] or from L-arginine (L-Arg) by various nitric oxide synthase (NOS) enzymes. [143]Similar to ROS, the biological effects of RNS are also concentration-dependent.For example, •NO can promote solid tumor angiogenesis at a concentration below 0.1 μm. [144]owever, at high •NO concentration(>1 μm), it can form highly oxidizing ONOO − , which induces DNA damage, cell apoptosis, and eventual tumor regression. [145]Therefore, precise cancer therapy can be realized by enhancing RNS generation (Figure 5a).
As the source of all RNS in the biological systems, NO has a short half-time and limited diffusion distance within the body (<10 s and <1000 μm, respectively) after its generation. [107]oreover, the biological function of NO is highly dependent on its concentration. [149]Therefore, various stimuli-responsive nanomaterials have been designed to precisely regulate NO generation and achieve satisfactory therapeutic outcomes.Generally, in situ NO generation in the desired tissues can be attained through either endogenous or exogenous triggers, such as ROS, GSH, and hyperthermia.These strategies for NO generation can be classified into two categories: 1) Synthetic NO donors, such as organic nitrates, nitrites, metal-NO complexes, nitrosamines, S-Nitrosothiols, and diazeniumdiolates, [150] are loaded into nanomaterials [151] and triggered to release NO by an exogenous or endogenous stimulus such as transition metal ions, [152] pH, [153] GSH, [154] enzymes, [155] and nanozymes. [156]For example, Wang et al. have developed S-nitrosylation polymers and their nanoassemblies as a NO nanogenerator. [146]The NO nanogenerator enables controlled NO release in response to tumoral GSH, inducing immunogenic cell death (ICD) (Figure 5b).In addition, NO donors-based strategy could be combined with chemotherapeutic drugs to overcome multidrug resistance and enhance the therapeutic effect.For instance, Ji et al. synthesized mitochondria-targeting BO (-CD-DOX-NO-DA NPs) containing both a mitochondria-penetrating peptide PEG-(KLAKLAK) 2 CGKRK and -cyclodextrin-based prodrugs The approach of stimuli-responsive nanomaterials to induce RNS generation.b) Nanoassemblies of S-nitrosylation polymers as a NO nanogenerator enable controlled NO release in response to tumoral GSH, inducing immunogenic cell death (ICD) to enhance immunotherapy.Reproduced with permission. [146]Copyright 2022, American Chemical Society.c) Schematic illustration of a nanozyme-based NO generator, CeO 2 -AT to specifically produce NO under the catalysis of NOS in mitochondria of cancer cells for selective oncotherapy.Reproduced with permission. [147]Copyright 2022, Springer Nature.d) A supramolecular Pt/NO prodrug nanoassemblies strategy realized ONOO − -potentiated chemotherapy of liver cancer.Reproduced with permission. [148]opyright 2021, American Chemical Society.
for DOX and NO. [157]When taken up by cancer cells, such specific mitochondria-targeted delivery of NO proves crucial in inducing mitochondria dysfunction through facilitating mitochondrial membrane permeabilization and downregulating ATP level, which can inhibit P-glycoprotein-related bioactivities and formation of tumor-derived microvesicles to combat drug resistance and cancer metastasis.2) Endogenous NO is generated from the nitrogen atom of the terminal guanidine group of arginine in the presence of NOS under physiological conditions or through the oxidation of arginine by some ROS, such as H 2 O 2 . [158]Inspired by the biosynthesis of endogenous NO that utilizes L-Arg as a natural donor, [159] a variety of L-Arg-based nanosystems have been developed. [160]However, the limited oxidation of TME and the availability of L-Arg pose challenges in generating sufficient NO, limiting the therapeutic effectiveness of L-Arg-based nanosystems.To enhance the NO generation in tumor tissues, elevating the oxidation of TME and the supplement of L-Arg are effective strategies for enhancing antitumor activity.For example, the combination of arginine and GOx is an effective strategy for facilitating NO generation. [161]Enzyme mimics that accelerate the production of O 2 and 1 O 2 can also enhance NO generation from arginine. [147,162]For instance, Sun et al. have synthesized a nanozyme-based NO generator, cerium oxide (CeO 2 )-AT, [147] motivated by the fact that mitochondria of cancer cells express excessive NOS.After endocytosis into cancer cells, the generator triggered the production of NO molecules within the mitochondria of cancer cells through the catalytic action NOS, thus disrupting the mitochondrial respiratory chain of tumor cells and further inducing cell apoptosis (Figure 5c).In addition, the generator with CAT-like activity can catalyze H 2 O 2 to produce O 2 , which can promote the generation of NO and improve the performance of NO gas therapy.Similarly, Chen et al. have also designed an endogenous generator that concurrently produces 1 O 2 and NO by integrating the arginine with Ru nanozyme.162b] ONOO − , as a product of the reaction between NO and •O 2 − , is far more toxic than most free radicals, including •OH. [163]Such toxicity is attributed to its direct oxidation capacity as well as the formation of carbonate (CO 3 • − ) and nitrite (•NO 2 ) radicals through radical-mediated nitration reactions, [164] which induce a variety of detrimental effects, including DNA strand breakage, disruption of membrane structure by lipid peroxidation, direct damage to mitochondria, and the promotion of cell death. [165]ue to the transitory lifetime and limited diffusion distance of NO and •O 2 − , achieving precise control over their simultaneous production within the same cellular locale presents a significant challenge for the efficient generation of ONOO − .Deng et al. addressed this challenge by devising a tailored approach involving supramolecular Pt/NO prodrug nanoassemblies for ONOO −potentiated chemotherapy of liver cancer. [148]The supramolecular prodrug nanoassemblies were prepared by supramolecular self-assembly of the host block copolymer and the two guest prodrugs: NO donor and Pt prodrug.Upon targeted delivery into liver cancer cells through endocytosis, both Pt(II) and NO are intracellularly released to produce a highly toxic ONOO − , which in turn downregulates the levels of GSH reductase and xeroderma pigmentosum group A, thus synergistically decreasing detoxification processes and blocking the repair of DNA damage caused by Pt-based chemotherapy (Figure 5d).
Generation of Other Reactive Oxidative Species: Cancer treatment based on oxygen-free radicals has emerged as a promising clinical approach.However, most solid tumors contain severely hypoxic areas, [166] which prevent the formation of oxygen-free radicals.A feasible solution to address this issue involves the delivery of a precursor compound into the tumor, allowing for the in situ generation of radicals upon activation.Most free radicalgenerating precursors are chemically unstable due to their susceptibility to in vivo decomposition, which inevitably increases the side effects.Herein, we provided an overview of the anticancer effects of carbon-centered radicals (CO, alkyl radicals) and sulfate radicals, and the principles for the design of stimuliresponsive nanomaterials to precisely control the free radicals release in the tumor site.
As an endogenous signaling molecule, carbon monoxide (CO) can inhibit the cytochrome of NOXs and the mitochondrial respiratory chain by interacting with heme, thus increasing the production of ROS and inducing mitochondrial damage and dysfunction in cancer cells. [167]Therefore, the use of CO as a therapeutic agent for cancer has already been extensively studied. [168]A growing number of CO-releasing molecules, including metal carbonyl compounds, aldehydes, oxalates, boron carboxylates, and silacarboxylates, have been developed to carry and release CO in response to various endogenous stimuli, [169] such as mild acidity, [170] overproduced H 2 O 2 , [171] and reductive molecules (e.g., GSH) [172] or enzymes. [173]However, therapies relying on CO-releasing molecules face many challenges, including limited stability in biological environments, potential toxicity, lack of precise spatial-temporal control, and low cellular uptake and tissue accumulation.The use of stimuli-responsive nanomaterials could realize high CO payloads, lower dosage, enhanced biosafety, and targeted CO delivery to specific sites, thus they have been designed to settle aforementioned challenging problems. [174]For example, He's group has used mesoporous silica nanoparticles and MOF nanocomplex to encapsulate metal carbonyl compounds to realize the intratumoral acid-triggered release of CO through a Fenton-like reaction. [175]As shown in Figure 6a, small-sized MnO 2 nanoparticles and FeCO were sequentially loaded into mesoporous silica nanoparticles.Within the intratumoral acidic environment, MnO 2 produced Mn 2+ and •OH, subsequently triggering CO release from Fe 3 (CO) 12 .175c] Alkyl radicals can be generated from some azo-based initiators, requiring only mild heat stimulus without oxygen participation.These radicals effectively eradicate cancer cells by directly oxidizing cellular components or reacting with oxygen to generate O 2 -containing free radicals, such as alkoxy and peroxy groups.Leveraging the unique biological feature of the mitochondrial matrix, which maintains a temperature ≈6-9 °C higher than the surrounding cytoplasm, a core-shell structure drug delivery system targeting mitochondria was developed.This system involved loading 2,2′-azobis[2-(2-imidazolin-2-yl) propane] dihydrochloride (AIPH) into triphenylphosphine-modified mesoporous silica nanoparticles, which were then encapsulated by folic acid tagged pH-sensitive liposomes containing docetaxel. [176]Once delivered to mitochondria, AIPH generates alkyl radicals in the high-temperature environment, leading to cause oxidative damage and enhancing the cellular apoptosis process (Figure 6b).
Sulfate radicals (•SO 4 − ), as intermediates generated by activation of persulfate or peroxymonosulfate (PMS), have a longer half-life time (30-40 μs) and higher redox potential (2.5-3.1 V) than most of the free radicals. [178]Therefore, combining them with oxygen radicals becomes an advisable synergistic strategy for cancer therapy.In a pioneering study, Liu et al. have prepared phospholipid-coated Na 2 S 2 O 8 nanoparticles (PNSO NPs) for the in situ generation of Na + and S 2 O 8 2− through gradual degradation. [177]These species can then transform into toxic •SO 4 − and •OH, regardless of the levels of H 2 O 2 and pH within the TME.The substantial production of Na + , toxic •SO 4 − and •OH results in a surge of osmolarity, rapid cell rupture and lysis, and caspase-1-related pyroptosis.Furthermore, these PNSO NPs can regulate the immunosuppressed TME and activate systemic antitumor immune responses to combat tumor metastasis and recurrence (Figure 6c).
In this section, we presented an overview of recent advances in the design and fabrication of responsive nanomaterials to induce excessive reactive oxidative species by taking advantage of TME such as low pH, high GSH concentration, excessive H 2 O 2 , severe hypoxia, etc.A variety of strategies and nanomaterials are extensively exploited for ROS/RNS generation for tumor suppression.These nanosystems exhibit high efficiency, good targeting capacity, stimuli sensitivity, and biocompatibility, thereby making them highly attractive.However, few ROS/RNS-generating nanosystems have been approved in clinics or clinical trials.The obstacles include unclear pharmacokinetics of these nanomaterials in the human body, difficulty of large-scale, repeated preparation, etc.The responsive nanomaterials to induce oxygen-free oxidative species have not yet been widely developed.Considering that most solid tumors contain severely hypoxic areas, this strategy may be a promising research direction.In addition, TME differs significantly from the normal cellular microenvironment and is not limited to the above points.Searching for new TMEresponse features will help create more promising nanomaterials and develop more TME-response strategies.

TME-Responsive Nanomaterials that Generating Reactive Oxidative Species Under External Stimuli
External energy fields such as light, X-ray, and US irradiation have the potential to enhance the generation of ROS and Copyright 2019, Springer Nature.b) AIPH/MSN-TPP@Lipo/DTX-FA NPs released alkyl radicals in mitochondria at high temperatures and then combined with DTX to enhance the cell apoptosis process.Reproduced with permission. [176]Copyright 2020, Elsevier.c) PNSO releases sulfate radicals under TME and triggers ROS storms to lead to immunotherapy through the ICD process.Reproduced with permission. [177]Copyright 2020, American Chemical Society.Abbreviations: TPP, 5,10,15,20-Tetraphenylporphyrin.
improve anti-tumor effects.In response, numerous external stimuli-responsive nanomaterials have been rationally designed to achieve a precise and controlled generation of reactive oxidative species.These nanomaterials exhibit intriguing advantages, including minimal or non-invasiveness and specific spatiotemporal selectivity.According to the external stimuli response sources, they are roughly divided into light, X-ray, US, and other stimulation methods.
Light Stimuli-Responsive Nanosystems for Inducing the Generation of Reactive Oxidative Species: Due to its merits of spatiotemporal addressability, noninvasiveness, high specificity, and easy operation, phototherapy is considered to be a promising approach for inducing the generation of ROS.Moreover, as an external stimulus, light is also extensively used to control the release behaviors of other free radicals (including NO, CO, carboncentered radicals, and other radicals) at precise locations.
Light-Induced ROS Generation: In the field of nanomaterials that generate ROS by light, photodynamic strategy (photodynamic therapy: PDT) has extensively been used to treat various cancers. [179]Specifically, PDT works via two mechanisms: type I and type II reactions. [180]In type-I reactions, the charge transfer among photoexcited photosensitizers with triple oxygen ( 3 O 2 ) or water molecules leads to the generation of •OH and •O 2 − . [181]hile in type II reactions, the excited triplet of photosensitizers directly transfers its energy to 3 O 2 , yielding reactive 1 O 2 for therapeutic purposes. [182]To date, ≈20 photosensitizers have been commercialized or used in clinical trials.However, these available photosensitizers face several major challenges, such as limited tissue penetration depth, low targeting efficiency, restricted O 2 supply due to hypoxic microenvironments, and elevated levels of GSH.To overcome these grave challenges, transition metal coordination complexes, organic-fluorophores, and  [196] Copyright 2021, Wiley-VCH.b) Schematic illustration of the acidic TME-responsive ECPM nanohybrids for NIR-induced PDT and downconverted NIR-II imaging.Reproduced with permission. [200]Copyright 2020, American Chemical Society.c) Schematic illustration of DPP-NF NPs for controllable "on-off" release of NO under light/dark conditions.Reproduced with permission. [211]Copyright 2018, Wiley-VCH.d) Design of a controlled CO delivery nanomaterial for improving cancer therapy.Reproduced with permission. [214]Copyright 2020, American Chemical Society.e) Schematic illustration showing the controlled release and generation of alkyl radicals upon irradiation by NIR laser.Reproduced with permission. [217]Copyright 2017, Wiley-VCH.f) CNPPtCP/si(c-fos) generates oxygen-independent N 3 • for endo/lysosomal escape, accompanied with the subsequently released Pt(II) and si(c-fos) for synergistic cancer therapy.Reproduced with permission. [228]Copyright 2020, American Chemical Society.g) BSO@Fe 3 O 4 -Sch nanocomposites produce •SO 4 − under the attacking of abundant •OH, which is produced through Fenton reaction trigged by NIR laser.Reproduced with permission. [229]Copyright 2021, Wiley-VCH.h) Schematic diagram of UCSAP synthesis and •Cl generation for hypoxic tumor therapy.Reproduced with permission. [231]Copyright 2020, Wiley-VCH.
semiconducting polymer nanoparticles have been extensively exploited as attractive PDT agents. [183]Inorganic nanomaterials, including TiO 2 , [184] ZnO, [185] g-C 3 N 4 , [186] Pt-CDs, [187] Bi and its oxides, [188] etc., have emerged as promising ideal photosensitizer candidates for PDT.Various nanosystems-based solutions have been developed to improve the PDT effect.To enhance the effects of PDT in deep tissues, photosensitizers with a high two-photon absorption have been designed, including Au nanoclusters, [189] quantum dots, [190] carbon nanomaterials, [191] silica nanoparticles, [192] polymer nanoparticles, [193] or other nanoparticles. [194]Moreover, applying light conversion nanomaterials which can convert NIR light and X-rays with high tissue penetration depth into visible light has emerged as a new technology for activating photosensitizers for deep-tissue PDT. [195]For instance, Chen et al. have used multiphoton excited up-conversion nanoparticles (UCNPs) as energy transducers to convert NIR light into blue light through the fluorescence resonance energy transfer (FRET) strategy, [196] thus realizing oxygen-independent •OH bursts via H 2 O-mediated photocleavage of blebbistatin [197] (Figure 7a).Chen et al. have engineered a nanosystem that consists of SrAl 2 O 4 :Eu 2+ core with double-layer silica coats.Photosensitizer merocyanine 540 is embedded into the mesoporous silica layer. [198]Upon irradiation, SrAl 2 O 4 :Eu 2+ core converts X-rays to visible light photons, thus activating merocyanine 540 to trigger the production of 1 O 2 for PDT therapy.The substantial overlap between the X-ray-excited optical luminescence of SrAl 2 O 4 :Eu 2+ and the excitation wavelength of merocyanine 540 augments the effectiveness of photosensitizer activation with low-energy X-rays.Moreover, persistent luminescence nanoparticles that maintain long afterglow luminescence after the removal of the light source are an emerging class of luminescent materials as transducers for X-ray-induced PDT. [199]ang's group has presented a strategy for depth-independent and repeatable PDT by using low-dose X-ray-activatable W(VI)-doped ZnGa 2 O 4 : Cr NPs as an excitation source and Zn(II) phthalocyanine tetrasulfonic acid as photosensitizer.199b] The combination of the deep penetration of X-rays and the specific targeting of PDT minimizes the side effects caused by X-ray irradiation with ultra-high doses and overcomes the limitations of shallow light irradiation penetration.Furthermore, Photosensitizers have also been conjugated with photothermal nanomaterials to achieve PTT-enhanced PDT owing to the heat produced by PTT can increase blood flow and thus improve oxygen supply to enhance PDT efficiency. [200]o enhance the specific distribution of photosensitizers in tumor sites, several activatable nanomaterials responsive to the acidic TME were recently engineered for tumor-specific delivery of the photosensitizer and enhanced PDT against tumors. [201]ur group has also reported a pH-responsive i-motif DNAmediated strategy to achieve selective tumor accumulation of photosensitizer and enable deep-tissue PDT. [202]As shown in Figure 7b, the virus-like erbium-based rare-earth nanoparticles decorated copper doped MOFs nanohybrids were modified with pH-responsive i-motif DNA strands via the metal-phosphate coordination interactions.Within the acidic TME, the i-motif DNA strands can form quadruplex structures, resulting in the assembly of nanohybrids and selective tumor accumulation.Under low-energy photon excitation, the emitted visible photon from the ErNPs could be easily harvested by the porphyrin ligands in copper-doped MOFs for NIR-induced photodynamic therapy.
Light-Controlled Release of NO: Various NO donors with photostability, such as S-nitrosothiols, metal-NO complexes, Nnitrosamines, nitrobenzene, etc., have been developed for NO release.However, most light-responsive NO donors are activated by ultraviolet and visible light, thus limiting their tumor application.To achieve deep-tissue penetration, an increasing number of nanocarriers, including carbon nanostructures, [203] silica NPs, [204] TiO 2 NPs, [205] noble metal NPs, [206] metal sulfide NPs, [207] Fe 3 O 4 NPs, [208] copolymers, [209] and UCNPs [210] have been exploited to encapsulation of photoactive NO donors to enable photocontrolled release of NO.210b] Guo et al. have reported ruthenium nitrosyl and TPP-functionalized N-GQD as a mitochondria-targeting nanoplatform.203a] L-Arg/Dox-loaded liposomal Au@CuS yolk-shell nanoparticles were designed to sequentially release NO and Dox for multidrug resistance (MDR) cancer therapy under 808 nm laser irradiation. [206]The sequential release of NO and Dox could significantly inhibit P-gp expression and enhance Dox accumulation in Dox-resistant MCF-7/ADR cells.Wang et al. have used the covalent bonding of a NIR-responsive NO photodonor (4-nitro-3-trifluoromethylaniline, NF), and pH-sensitive unit (dimethylaminophenyl-) and diketopyrrolopyrrole core to achieve 1 O 2 generation and NO release under weakly acidic conditions of lysosomes (pH 4.5-5.0)(Figure 7c). [211]ight-Controlled Release of CO: The photolytic features of metal carbonyl complexes have been integrated with nanocarriers for the photocontrolled release of CO. [212] For instance, graphene oxide nanosheet was employed as carriers for Mncarbonyl and ruthenium carbonyl to achieve NIR-responsive release of CO. [213] Zhang et al. have designed a controlled CO delivery nanomaterial by encapsulating iron pentacarbonyl (Fe(CO) 5 ) within an Au nanocage under an oxygen-free atmosphere [214] (Figure 7d).Under aerobic conditions, Fe(CO) 5 is transformed into iron oxide on the surface of the Au nanocage in a controllable manner, efficiently preventing leakage and oxidation of the encapsulated Fe(CO) 5 , thereby enhancing its stability and biocompatibility.Under NIR irradiation, the photothermal effect of Au nanocages triggers the decomposition of the encapsulated Fe(CO) 5 to generate CO and iron resulting in remarkable synergistic effects in cancer cells, as presented by inducing mitochondrial damage, autophagy, and the destruction of autolysosome.In addition, inspired by CO 2 natural capture and transformation, Zhang's group has prepared an innovative CO-producing photocatalyzed nanomaterial, Ag 3 PO 4 doped carbon-dot-decorated C 3 N 4 nanoparticles functionalized with histidine-rich peptide, which could transform tumor endogenic CO 2 to CO under the precise control of body-penetrating 630 nm light, significantly enhancing chemotherapy toward prostate cancer cells. [215]ight-Responsive Generation of Carbon-Centered Radicals: The thermal decomposition of azo initiators to produce alkyl radicals is constrained at physiological temperatures.The NIRbased photothermal effect provides a feasible solution to accelerate the generation of free radicals.In 2017, Zhang et al. presented a novel therapeutic strategy for NIR-induced generation of alkyl radicals. [216]An initiator of 2,2-azobis[2-(2imidazolin-2-yl)propane] dihydrochloride (AIBI) was loaded in the hollow interiors of gold nanoclusters (AuNCs), followed by coating of thermal-responsive poly(N-isopropylacrylamide-coacrylamide) as a gatekeeper to form AIBI@AuNC-copolymer. Under NIR irradiation, the plasmonic heating of AuNCs facilitated the decomposition of AIBI to generate alkyl radicals, as well as causing the phase transition of the copolymer to release the blocked alkyl radical.Similarly, Xia et al. have also demonstrated the use of AuNCs filled with AIPH (the same as AIBI) along with phase-change material to achieve a controlled generation of free radicals for cancer therapy [217] (Figure 7e).Since the photothermal efficiency of Au nanocages is not satisfactory, nanomaterials with higher photothermal efficiency were conjugated to radical initiators to generate alkyl radicals, including porphyrinbased organic nanoparticles, [218] carbon-coated iron carbide (Fe 5 C 2 ) nanoparticles, [219] CuFeSe 2 @BSA, [220] MoS 2 @PCM, [221] , Bi 2 Se 3 , [222] , and others. [176,223]Another carbon-centered radical, trifluoromethylation radicals (•CF 3 ), possesses a stronger electrophilic ability than ROS when it comes to attacking unsaturated bonds and most amino acids in organisms. [136]Zheng et al. have presented a triaryl methane/CF 3 SO 2 Na-based liposomal nanosystem that efficiently promotes ferroptosis and apoptosis to treat drug-resistant tumors. [224]Under 630 nm light irradiation, the nanosystem generated •O 2 − , •CF 3 , and SO 2 within drug-resistant tumor cells.The generated substance is coordinated to disrupt the redox balance of the drug-resistant system by increasing oxidation and decreasing reduction, prompting the accumulation of a large amount of lipid peroxidation to induce ferroptosis.
Light-Responsive Generation of Other Radicals: Photoactive platinum azido complexes exhibit remarkable stability in the absence of light.However, upon irradiation, they can be activated to generate azidyl radicals (N 3 •), •OH, and anticancer-active Pt(II) species.This property underscores their potential as photosensitizers for O 2 -independent PDT. [225]Based on the unique characteristics of platinum azido complexes, Huang et al. have bound a Pt(IV)-azide complex and demethylcantharidin to form a multifunctional single-drug, which was further conjugated to an amphiphilic block copolymer to form a micelle. [226]Under ultraviolet A irradiation and endosomes/lysosomes environments, the micelle was activated to release Pt, N 3 •, and demethylcantharidin, showing great potential for overcoming acquired tumor resistance to cisplatin.Furthermore, their group further encapsulated Pt(IV)-azide complexes in various nano/microcarriers for tumor therapy. [227]For example, they connected the photoreactive Pt(IV) azido precursor drug to siRNA for synergistic cancer therapy [228] (Figure 7f).− when exposed to abundant •OH (Figure 7g). [229]The combination of dual-radical production, Fe 3 O 4 nanocrystal-mediated PTT, and GSH depletion causes an imbalance in the intracellular redox state, eventually leading to cell death.Besides, Ding et al. have prepared peroxymonosulfate-loaded hollow mesoporous CuS NPs with polyethyleneimine modification for photothermalenhanced sulfate radical nanotherapy in melanoma.Upon irradiation with 1064 nm light, the copper ions are initially released from CuS, followed by the effective activation of PMS to generate toxic •SO 4 − and •OH, which is independent of the O 2 and H 2 O 2 content within the tumor. [230]hlorine radical (•Cl), with an oxidation potential (2.47 V) second only to •OH, has a longer lifespan and diffusion distance, and also shows stronger and faster reactivity to certain electronrich substances by nucleophilic addition (Table 1).Since there are many nucleophilic atoms and functional groups in biological molecules, •Cl holds enormous potential for highly efficient tumor eradication via •Cl-induced cellular stress. [232]Bu's group [231] has designed the •Cl nano-generators with a SiO 2 -coated UCNPs interior and decorated with Ag 0 /AgCl hetero-dots on the out-side.As illustrated in Figure 7h, upon NIR irradiation, the shortwavelength emission light UCNPs catalyze Ag 0 /AgCl to generate •Cl independent of O 2 /H 2 O 2 , which induces cellular •Cl stress and DNA double-strand breaks in tumor cells.
In summary, as one of the most common excitation forms of external energy fields, light stimuli-responsive nanomaterials capable of generating reactive oxidative species have matured significantly.The clinicalization of photosensitizers has propelled extensive research into light-induced ROS nanomaterials.Impressive strides have been made in the utilization of light to activate novel free radicals such as CO, •SO 4 − , and •Cl.Future efforts in this area should focus on enhancing the penetration ability of light (extending into the NIR-II region and beyond) while maintaining efficient generation of oxidative species, aiming to achieve applications that are closer to clinical.
X-Ray Stimuli-Responsive Nanosystems for Inducing Reactive Oxidative Species Generation: X-rays have a higher penetration in the human body than NIR light. [233]More importantly, X-ray irradiation exposure leads to the radiolytic formation of various ROS, including •O 2 − , making X-Ray an ideal excitation source for ROS generation. [234]A great mass of multifunctional nanomaterials has been developed for augmenting X-rays-induce ROS generation inside tumor tissues and elevating the antitumor therapeutic impact.They can be commonly divided into three types: 1) Light conversion nanomaterials for X-ray-induced PDT.These nanomaterials can convert X-rays into visible lights to activating photosensitizers, including scintillating NPs, [18b,235] persistent luminance NPs, [199] aggregation-induced emission (AIE) NPs, [236] and nanoscale MOFs. [237]Recently, Wang et al. have prepared a class of organic phosphorescence nanoscintillators based on 9,9'-(6iodophenoxy-1,3,5-triazine-2,4-diyl)bis(9H-carbazole), which act in a dual capacity as scintillators and photosensitizers.This approach demonstrated great potential for deep tumor treatment with a low dose of 0.4 Gy and negligible adverse effects. [238]2) High atomic number (high-Z) elements-based nanomaterials.They respond to ionizing radiation and enhance the therapeutic effect of radiotherapy, including noble metals (Au, Ag, Pt, etc.), rare earth elements (Gd, Ho, Tm, etc.), and other heavy elements (Hf, Ta, W, Bi, etc.). [239]Moreover, high-Z elements can be integrated with semiconductor nanomaterials to further enhance the therapeutic effect of radiotherapy. [240]For instance, Wang et al. have used Bi and I as High-Z elements to fabricate ultrasmall semiconductor materials to photocatalytic generate ROS for radiosensitization. [241]However, previous studies have shown that DNA repair occurs within the first 5 min after exposure to X-ray irradiation, and then cells activate the DNA damage response (DDR) pathway to repair damaged DNA. [242]PTT can inhibit DNA repair, improve the permeability of NPs in tumor cells, and boost the O 2 perfusion to sustain O 2 -dependent therapy.Therefore, many multifunctional nanoplatforms have been designed to achieve PTT-enhanced radiotherapy. [243]For example, our group has utilized W 18 O 49 nanospheres to generate intracellular 1 O 2 and •OH under X-ray irradiation, resulting in three-times enhancement in the inhibition of 4T1 cell proliferation. [244]Moreover, to increase O 2 concentrations in hypoxia tumor tissues, oxygen-carrying and oxygen-generating strategies have been also employed to improve nanomaterialssensitized radiotherapy.Gu's group synthesized SnS 2 @Fe 3 O 4 nanocomposite consisting of SnS 2 nanoplates and Fe 3 O 4 quantum dots and proposed a concept of X-ray-facilitated redox cycling of peroxidase-like nanozyme for high-efficiency and persistent ROS generation in tumor therapy. [245]3) Organic small molecule-linked nanomaterials.For example, Xie et al. have reported a potassium iodide NP-based radiosensitizer.By employing the Na + /I − symporter for iodine delivery and radiosensitization, they achieved complete tumor eradication in 80% of treated animals without inducing additional toxicity [246] (Figure 8a).
X-rays are also used as external stimuli to penetrate the biological tissues for RNS generation. [247]The simultaneous production of NO and •O 2 − in the same region presents a significant challenge for the efficient generation of ONOO − .To solve this issue, Gu et al. have introduced theranostic scintillating nanoparticles containing Ce-doped LiLuF 4 and UV-responsive NO donors Roussin's black salt. [248]The Ce-doped LiLuF 4 can not only act as radiosensitizers for enhancing the yield of ROS, but also convert X-ray into UV light to activate the photoactive Roussin's black salt to release NO (Figure 8b).Such simultaneous release of NO and •O 2 − ensures the efficient, X-ray-controlled generation of ONOO − in tumors, consequently improving the radiotherapy efficiency by directly damaging DNA, inhibiting the PARP-associated DNArepair process and overcoming the hypoxia-associated resistance in radiotherapy.Bu's group [249] has proposed a strategy for the treatment of hypoxic prostate cancer based on X-ray-induced nitrosative stress, using an electrophilic ZIF-82.After internalization, pH-responsive ZIF-82-PVP nanoparticles release electrophilic ligands and Zn 2+ .The released electrophilic ligands were able to capture low-energy electrons derived from X-rays to produce NO 2 − , resulting in the inhibition of autophagy and increasing lethal nitrosative stress levels (Figure 8c).
US Stimuli-Responsive Nanosystems for Inducing ROS Generation: Compared with UV-vis light and NIR light, US offers superior tissue penetration capabilities (up to 20 cm depth by 1 MHz US wave) and can enhance vasculature permeability, which improves the cellular uptake and tissue permeability of therapeutic agents for the treatment of tumors.More importantly, low-intensity US irradiation can activate sonosensitizers concentrated in both tumors and tumor neovascular epithelial cells to produce a large amount of cytotoxic ROS such as 1 O 2 and •OH through the cavitation effect and sonoluminescence (called sonodynamic effect, SDT for short). [250]The ROS produced by SDT have the potential to destroy organelles of tumor cells and damage tumor vascular epithelial cells to release thromboxane, resulting in forming thrombus in tumor blood vessels and causing ischemic necrosis of tumor tissues.Thus, SDT has been widely explored as an external stimulus for inducing ROS generation.
Sonosensitizers are typically categorized into two main groups: inorganic sonosensitizers and organic sonosensitizers.Organic sonosensitizers include porphyrins and their derivatives, [251] photosensitizing substances, [252] etc. [253] However, organic sonosensitizers suffer from hydrophobicity, low bioavailability, and low accumulation in tumor sites, greatly restricting the efficacy of SDT.To address these challenges, researchers have extensively explored the use of both inorganic and organic nanocarriers.Like other O 2 -dependent therapies, insufficient oxygen supply in the hypoxic microenvironment also impacts the efficacy of SDT.Therefore, oxygen-carrying and oxygen-generating strategies have been widely used to moderate hypoxia and enhance the efficacy of SDT.For example, Chen and co-workers have designed an oxygen-self-produced SDT nanoplatform by using fluorocarbon-chain-functionalized hollow mesoporous organosilica NPs as carriers to load IR780.This nanoplatform effectively overcame the hypoxia-specific transportation barriers, supplied sufficient oxygen to hypoxic PANC-1 cells, especially upon exposure to US irradiation, and alleviated hypoxia. [254]Moreover, to reduce the toxicity of sonosensitizers to healthy tissues, TME-responsive smart nanomaterials have been developed for completing the effective accumulation at tumor sites.For instance, Wang et al. have reported new sonosensitizers consisting of porphyrins, chelated Cu 2+ , and PEG.The overexpressed GSH in the tumor reduces Cu 2+ to generate Cu(I), leading to precise SDT with minimized side effects. [255]norganic sonosensitizers can overcome the inherent drawbacks of organic sonosensitizers, with high hydrophilicity, better US stability, and lower phototoxicity.Among inorganic sonosensitizers, TiO 2 was the first inorganic sonosensitizer in SDT due to its ability to generate ROS via the dispersion of electrons and holes. [256]However, the wide bandgap and rapid electron-hole recombination (50 ± 30 ns) of pure TiO 2 result in a low yield of 1 O 2 .To tackle this problem, three types of strategies have been developed to suppress the recombination of electrons and holes: integrating TiO 2 with other multifunctional NPs to form nanocomposite, [257] enhancing its electroconductivity, [258] or creating an oxygen-deficient layer. [259]For example, Geng and colleagues have designed semiconductor p-n junctions using pyridine N-doped carbon dots (N-CDs) as a p-type semiconductor and oxygen-deficient TiO 2−x nanosheets as a n-type semiconductor.The rate constants of 1 O 2 and •OH generation by US-excited N-CDs@TiO 2−x p-n junctions are 4.3 and 4.5 times higher than those of TiO 2 , respectively, exhibiting enhanced sonodynamic performance and GSH depletion capability for complete eradication of malignant tumors without relapse. [260]Besides TiO 2 -based sonosensitizers, other inorganic semiconductor nanomaterials have also been explored as SDT sonosensitizers to promote ROS production, which include WO x , [261] VS 4 , [262] black phosphorus (BP), [263] heterostructures, [264] TiH 1.924 nanodots, [265] and defects/vacancies-containing nanomaterials, [266] layered double hydroxides (LDHs), [267] etc.266b] The specific coordinatively unsaturated Zn-N active site on the surface of ZIF-8 greatly facilitates ROS generation through enhancing the electron transfer to the lower unoccupied molecular orbital (LUMO) via ligand to metal charge transfer (LMCT) under US irradiation, while the release of Zn 2+ interrupts zinc homeostasis.Moreover, SDT still suffers from impaired efficacy in the treatment of most tumors due to insufficient oxygenation.Considerable efforts have been devoted to selfgeneration of O 2 at tumor sites.Sun et al. have synthesized Fedoped multivalent manganese oxide nanoparticles via a vacancy engineering strategy. [268]These nanoparticles not only limited the recombination of electron-hole pairs, but also absorbed enough oxygen on the surface, thus bestowing nano-sonosensitizers with a hypoxia-irrelevant 1 O 2 generation ability (Figure 8d).
In addition to various strategies for increasing ROS generation in SDT, US irradiation has been used to increase the generation of other radicals, [269] including RNS, carbon-centered radicals,  [246] Copyright 2021, American Chemical Society.b) Schematic illustration showing the synthetic route to RBS-T-SCNPs and the X-ray-controlled ONOO − generation for on-demand cancer radiotherapy.Reproduced with permission. [248]Copyright 2018, Wiley-VCH.c) Schematic illustration of the electrophilic ligands released by ZIF-82-PVP generate and sulfate radicals.In 2010, Gu's group [270] designed NO microreactors for US-triggered NO generation and MRI localization of the reactor by using a double emulsion method to simultaneously encapsulate L-Arg and superparamagnetic nanoparticles.Jin et al. have also developed a US-triggered NO release nanoplatform by encapsulating a NO-releasing molecule into a rattle-type nano-carrier of superparamagnetic iron oxide/hollow mesoporous silica nanoparticles, and the amount of released NO can be finely tuned by adjusting the power of the US. [271]Guo et al. have reported a US-responsive drug delivery system by encapsulating an AIPH-loaded C-TiO 2 hollow nanoshells functionalized with a platelet membrane, which can efficiently produce ROS and alkyl radicals upon US excitation. [272]Du et al. have developed a smart nanoplatform for multimodal synergistic therapy using MoS 2 nanoflowers as both a sonosensitizer and nanozyme, CF 3 SO 2 Na as a CF 3 • precursor and cisplatin as the chemotherapeutic drug. [137]Under US irradiation, MoS 2 nanoflowers catalyze the decomposition of H − production, which remarkably enhanced the therapeutic efficacy of SDT for cancer. [273]ther Stimuli-Responsive Nanosystems for Inducing Reactive Oxidative Species Generation: Besides the three common external stimuli-responsive nanosystems mentioned above, various nanomaterials based on electrical field, magnetic field, and microwave have also been developed for inducing ROS generation.
Electrodynamic therapy (EDT) refers to the use of an external electric field to generate ROS to destroy tumor cells. [274]Nanomaterials can amplify the ROS generation ability of EDT.In 2019, Liu's group discovered that the surface of Pt NPs reacts with H 2 O and Cl − to generate •OH under the action of an electric field [275] (Figure 8f).In vivo experiments showed that this strategy could eliminate surface tumors larger than 500 mm 3 .After that, the combination of Pt-based EDT with starvation therapy and chemotherapy has been tried to improve ROS-based dynamic therapy. [276]Moreover, the high level of Cl − will promote the production of •OH in EDT.Introducing exogenous Cl − to tumor tissues or transporting extracellular Cl − into cancer cells using a chloride ion transporter has been shown to significantly improve ROS generation in EDT. [277]s a noninvasive, controllable, and penetrating external stimulus, magneto-thermal therapy (MHT) has been widely applied in clinical tumor ablation. [278]However, it is difficult to achieve complete tumor ablation in the site by heat alone due to the uneven enrichment of magnetic nanomaterials in tumors.To address this issue, Shi's group has proposed a novel magnetoelectrodynamic therapy method (MEDT) by employing CoFe 2 O 4 -BiFeO 3 (CFO-BFO) magnetoelectric nanoparticles for •OH and •O 2 − generation. [279]Under the stimulation of alternating magnetic fields, the magnetostrictive CFO core and the multiferroic BFO shell produce magneto-generated electrons and holes, ultimately facilitating the generation of ROS for effective tumor therapy.
Microwave (MW) irradiation, characterized by its low tissue invasiveness and high tissue penetrability, stands as an ideal external stimulus for cancer therapy. [280]However, the energy in the MW (400-2500 MHz) is relatively low (10 −3 eV), thus ablating tumors with MW irradiation have a limited diameter of 3 cm due to the restriction of heat transmission diameter. [281]o address the above problems, some MW-sensitive nanomaterials have been investigated to enhance MW-mediated cancer therapy. [280]For instance, Meng's group has revealed that liquid metal (LM) supernanoparticles (SNPs) activated by MW irradiation can generate •OH and •O 2 − , which are produced by the electron transfer from Ga to the water and oxygen adsorbed in the mesopores of SNPs. [282]Additionally, some novel nanomaterials have been also explored to generate 1 O 2 under MW irradiation, including Cu 2 ZnSnS 4 nanocomposites, [283] g-C 3 N 4 QDs, [284] TiMOF@Ni NPs@covalent organic frameworks (COF), [285] Bi-Mn-MOF@COF-based BMCAP [286] (Figure 8g).However, the mechanism of these nanomaterials generating 1 O 2 under MW remains unclear.One possible explanation [285] is that they produce "hot spots" under MW irradiation, resulting in point defects or weak surface bonds, where the reaction substrate (H 2 O, O 2 , H 2 O 2 ) is adsorbed to generate ROS.
Collectively, with rationally designed multifunctionality, enormous responsive nanomaterials have been developed for generating reactive oxidative species under different external stimuli to disrupt intracellular redox homeostasis, ultimately inducing apoptosis, autophagy, necrosis, and ferroptosis for cancer therapy.A large number of in vivo examples demonstrated that these types of stimuli-responsive nanosystems have made outstanding contributions to improving the efficiency of ROS/RNS-based therapy, driving collaborative treatment of cancer, and circumventing some drug resistance mechanisms.However, many questions need to be solved before their clinical applications, such as the poor controllability and low specificity of nanosystems, long-term safety, and the power and accuracy of the external stimuli generator.Fortunately, several nanomedicines have been approved in clinic or clinical trials.For example, the iron oxide nanoparticle-based nanomedicine (NanoTherm) won approval in Europe for the treatment of glioblastoma.A singlearm study in two centers demonstrated that thermotherapy using intratumoral magnetic nanoparticles in conjunction with a reduced radiation dose is safe and effective and leads to longer NO 2 − under X-ray irradiation to enhance lethal nitrosative stress.Reproduced with permission. [249]Copyright 2021, Wiley-VCH.d) Fe-doped multivalent manganese oxide nanoparticles (FDMNs) via vacancy engineering strategy as a hypoxia-irrelevant sonosensitizer for enhanced sonodynamic therapy.Reproduced with permission. [268]Copyright 2022, Elsevier.e) Schematic of •OH, •CF 3 , and SO 2 formation process of HA@MoCF 3 Pt NPs and mechanism of inhibiting cisplatin resistance under US irradiation.Reproduced with permission. [137]Copyright 2022, Elsevier.f) Schematic illustration of •OH generation by PtNPs under the electric field.Reproduced with permission. [275]Copyright 2019, Wiley-VCH.g) Schematic diagram of BMCAP generates ROS and heat for tumor therapy under MW irradiation.Reproduced with permission. [286]Copyright 2022, Elsevier.
OS-2 compared to conventional therapies in the treatment of recurrent glioblastoma. [287]The clinical pilot device study demonstrated that gold-silica nanoparticles (AuroShell particles) designed to absorb near-infrared light at wavelengths of high tissue transparency -mediated focal laser ablation was successfully achieved in 94% (15/16) of patients, with no significant difference in International Prostate Symptom Score or Sexual Health Inventory for Men observed after treatment, providing a highly localized light-based strategy for the treatment of prostate cancer. [288]HfO 2 nanoparticles, which are activated by externalbeam radiotherapy to generate oxygen free radicals, in a phase 2-3 clinical trial exhibit promising results clinically in patients with locally advanced soft-tissue sarcoma, without adding toxicity to the surrounding tissues. [289]These external stimuli-responsive nanosystems have shown great prospects for clinical translation.

Responsive Nanomaterials that Regulating Endogenous Oxidation Signaling Pathways
In addition to exploring nanomaterials designed for the overproduction of reactive oxidative species beyond the toxic threshold in tumor tissues, the abrogation of oxidative equilibrium by using exogenous nanomaterials to regulate endogenous oxidation signaling pathways provides an alternative therapeutic approach.
Targeting ROS-Generating Oxidases: Various xenobiotic, metabolic, and other physiological pathways are known to generate ROS.Several sources of ROS in cells and tissue have been identified, including mitochondrial electron transfer chain and enzymatic reaction of lipoxygenases, monoamine oxidases, xanthine oxidase, uncoupled NOS, NOX enzymes, and cytochrome P450 oxidases. [290]Therefore, targeting these ROS-generating oxidases through nanomaterials is an attractive modality to treat cancer.
NOX catalyzes the transfer of electrons from NADPH to molecular oxygen, resulting in the generation of O 2 • − and H 2 O 2 .The NOX family includes seven isoforms with different activation mechanisms: NOX1, NOX2, NOX3, and NOX5 produce •O 2 − while NOX4, DUOX1, and DUOX2 generate H 2 O 2 .NOXmediated ROS production plays a central role in various signaling pathways involved in the regulation of cell proliferation, differentiation, and apoptosis. [291]Numerous findings highlighted that NOX-induced ROS production provokes the acquisition of chemoresistance and contributes to cancer progression. [292]herefore, modulation of NOX expression, as well as their activity, is considered a new promising therapeutic approach for cancer treatment.There have been three attempts to target NOXmediated ROS production in cancer therapy.One methodology is using NADPH inhibitors or genetic silencing to suppress ROS-triggered somatic mutations and abnormal signaling pathways involved in tumor development and angiogenesis. [293]nfortunately, this approach does not currently show significant improvement due to the lack of highly specific and validated inhibitors for different NOX enzymes.Nanocarriers could significantly improve the effect of the inhibitors.For example, to overcome resistance to radiotherapy, Zhu et al. have developed a bioactive and CD44 targeted hyaluronic acid nanoparticle encapsulated with a NOX inhibitor GKT831, which had stronger inhibitory effects on ROS generation and cell prolifer-ation than GKT831 alone in cancer cells. [294]Another methodology is activating the expression of NOXs, enhancing NOXs activity, or inducing exogenous NOX NADPH mimics.For example, camptothecin, [295] cisplatin, [84,296] and Dox [297] can activate the membrane-spanning NOXs.A recent study showed that NADPH-producing and DOX-conjugating Escherichia coli (E.coli) exhibited enhanced antitumor efficacy. [298]He et al. have developed a nanoamplifier for CDT by coupling photoresponsive conjugated polymer NPs with ferrocene and cisplatin prodrug. [299]hang et al. have constructed a bifunctional biomimetic MOF-F bioorthogonal nanoplatform by coating cancer cell membranes on an F − absorbed MOF for prodrug activation and enhanced synergistic CDT. [295]After entering cells, the acidic lyso/endosome environment can disrupt the pH-sensitive metalligand bonds of MIL-53, subsequently releasing F − and Fe 3+ .The released F − is able to initiate the desilylation reaction in situ to activate OH-CPT.The generated OH-CPT is found to activate NOX to replenish intracellular H 2 O 2 , which further intensifies Fenton reaction (Figure 9a).In addition, some nanomaterials such as Pt NPs have NADH oxidase-like activity.Wu et al. have reported a NADPH-initiated cascade nanocatalytic system for ferroptotic tumor therapy by grafting Pt NPs onto MIL-101(Fe) framework possessing NOX/SOD-mimic activities and Fenton reaction capacity.The NOX nanozyme catalyzed NADPH oxidation to produce •OH/•O 2 − , which largely prevents the GSH regeneration and deactivates glutathione peroxidase 4 (GPX4), promoting lipid peroxidation for ferroptotic cell death. [300]Chong et al. have developed a cascade catalytic nanovaccine, metal-polyphenol networks adsorbed on the model antigen ovalbumin templates, in which the "Fe-GA catalytic unit" mimicked the ROS transfer process of NOX2. [301]onoamine oxidase A (MAOA) is a flavoprotein anchored to the mitochondrial outer membrane, that catalyzes the oxidative deamination of a number of biogenic and dietary amines to generate H 2 O 2 as a by-product in the process. [302]Recent studies have indicated that MAOA either has altered expression levels or exerts a regulatory effect in a variety of cancer types such as prostate cancer. [303]Therefore, suppressing MAOA/MAOB (isoenzymes) activity holds potential therapeutic prospects for tumor control. [304]Recently, Wu et al. found that AuNCs modified with cysteamine or 4-aminothiophenol, ≈1-3 nm in size, were found to have MAO inhibitor activity [305] (Figure 9b).The possible mechanism is that the proper size of AuNCs with ligands containing amino groups can bind tightly with the entrance to active sites of MAO, blocking the enzyme-substrate interaction.
Downregulating Nrf-2: As discussed in Part 2, Nrf-2 plays an important role in the regulation of cellular redox homeostasis. [306]rf-2 not only regulates the biosynthesis, utilization, and regeneration of three major antioxidant molecules, GSH, Trx, and NADPH, but also controls the production of ROS through mitochondria and NOXs. [307]Nrf-2 has multifaceted roles in cancer cells.Transient Nrf-2 activation has protective roles against carcinogenesis and cancer development, while permanent activation of Nrf-2 promotes malignant progression, chemo/radio resistance. [308]Several studies have provided evidence that preventing the permanent activity of Nrf-2 by peptide, small-molecule inhibitors, and small interfering RNAs (siRNAs) renders cancer cells susceptible to apoptosis and sensitizes cancer cells to chemo/radiotherapeutics. [309] Numerous  [295] Copyright 2022, American Chemical Society.b) Upper figure: MAO oxidase inhibitory ability of different concentrations of MAOI-like AuNC, bottom figure: Computational analysis of MAOI activity of size-dependent AuNCs.Reproduced with permission. [305]Copyright 2021, Elsevier.c) AGuIX nanoparticles amplify the ferroptosis system by inhibiting the Nrf-2-GSH-GPX4 signaling pathway.Reproduced with permission. [312]Copyright 2022, Springer Nature.
nanocarriers have been designed to improve the efficacy in targeting the TME and the combined administration of Nrf-2 modulators and conventional antineoplastic agents. [310]Apart from using nanomaterials as carriers for Nrf-2 modulators, recent studies demonstrated that several inorganic nanomaterials can directly downregulate Nrf-2.Hsieh et al. found that zerovalent iron nanoparticles enhanced GSK3/-TrCP-dependent degradation of Nrf-2 through activation of the AMPK/mTOR signaling pathway, thereby triggering ferroptotic cell death through excessive oxidative stress and lipid peroxidation. [311]un et al. designed the AGuIX nanoparticles that promoted not only radiation-induced DNA damage and apoptosis, but also ferroptosis by Nrf-2-GSH-GPX4-dependent pathway after irradiation may regulate the anti-ferroptosis system by inhibiting the Nrf-2-GSH-GPX4 signaling pathway (Figure 9c), enabling radiosensitization of gadolinium-based nanomaterials to kill triple-negative breast cancer (TNBC) cells. [312]n summary, redox signaling pathways play a vital role in regulating redox homeostasis.Redox signaling-based nanomedicine approach has emerged as a new platform for cancer therapy, where responsive nanomaterials modulate the expression of ROS-generating oxidases as well as their activity and downregu-late Nrf-2 to trigger excessive ROS formation.However, responsive nanomaterials regulating endogenous oxidation signaling pathways are rare.Moreover, the mechanism of nanomaterials regulating redox signaling is also fully understood.Therefore, a thorough understanding of nanomedicine-regulated redox signaling would inspire researchers to design and develop more novel nanomaterials that could be used as an alternative strategy for the treatment of cancer, where angiogenesis plays a vital role.In addition, an aberrant redox signaling cascade can be caused by several exogenous and endogenous factors and remain incompletely understood.A complete understanding of redox signaling pathways influenced cancer progression and chemo-or radioresistance would provide a guide for designing more nanosystems to regulate redox signaling pathways and improve cancer therapy.

Responsive Nanomaterials Inducing Reductive Damage
One characteristic of most cancer cells that distinguishes them from normal cells is that they exhibit higher ROS levels.However, to counteract excessive ROS production, some cancer cells and cancer stem cells constantly attempt to augment their antioxidant defense capacity, resulting in excessive production/accumulation of reducing equivalents, such as GSH, NADPH, NADH, and the free thiol group in the cysteine (Cys) residues of proteins, which drives the cells to reductive stress. [313]Currently, most therapeutic strategies such as chemotherapy and radiotherapy are intended to kill cancer cells through the generation of ROS, however, these strategies can eventually facilitate the development of drug/radioresistance. [314] Recent studies have revealed that excess reductive stress has similar deleterious effects as oxidative stress such as DNA damage, mitochondrial dysfunctions, reductions in cellular metabolism, and cell death via apoptosis or endoplasmic reticulum stress, thus leading to cellular dysfunction. [315]315a,317] Nanotechnology can also alter reductive stress in a tumor or its environment to suppress the proliferation and/or induce cell death (apoptosis) in tumor cells by delivering or generating additional reduction equivalents as well as reducing antioxidant mechanisms to indirectly cause oxidative damage.In this section, a series of responsive nanomaterials that cause reductive damage are summarized, including using responsive nanosystems for delivering antioxidants, producing high levels of reducing equivalents in the cellular environment or suppressing reductive stress to indirectly promote oxidative damage.

Responsive Nanocarriers for Delivery of Antioxidants
315b] Recent study demonstrated that the treatment of cancer cells with antioxidants under hypoxic conditions produces high levels of NAD(P)H before cell death, resulting in reductive stress instead of oxidative stress. [318]Based on these findings, several responsive nanocarriers have been designed to deliver antioxidants into the cancer cells to further strengthen the degrees of reducing stress. [319]For example, ascorbic acid (Vitamin C, AA for short) serves as an optimal reducing agent in human fluids, but its low transmembrane efficiency critically limits its antioxidative function within the cells.To address this problem, Tang et al. have designed poly(2-vinylpyridine)-polyethylene glycol-folic acid functionalized core-shell nanostructure of CdTe quantum dots with mesoporous silica coating, which can be used as a pHsensitive multifunctional drug nanocarrier with controllable release of AA under hypoxic environment. [320]AA was found to enhance the accumulation of reductive species, break the redox balance in HepG2 cells, and trigger the reductive stress to induce the apoptotic signaling pathway, thereby dealing minimum damage to normal tissues.Zhao et al. have constructed the reductive lipid NPs loaded with vinorelbine and reductive com-ponent Vitamin E, which can effectively reduce oxidative stress and significantly outperform free vinorelbine in preventing tumor progression. [321]

Responsive Nanosystems Generating Reducing Equivalents
Given that reductive stress can be useful against cancer, responsive nanosystems generating reducing equivalents to induce reductive stress in tumors or their environment without affecting other tissues have gradually been developed for cancer therapy.In this section, reductive stress-inducing nanosystems are classified according to reducing equivalents, such as hydrogen sulfide, hydrogen selenium, hydrogen gas, etc. (Table 3).
Generation of H 2 S: Hydrogen sulfide (H 2 S), an endogenous gasotransmitter, is derived from Cys and/or homocysteine in reactions catalyzed by cystathionine -lyase and cystathionine -synthase, or from 3-mercaptopyuvate in a reaction catalyzed by 3-mercaptopyruvate sulfurtransferase.H 2 S influences a broad range of physiological functions via post-translational modification of the thiol redox proteome, converting Cys thiols to persulfides. [322]Recent studies have revealed that, at higher concentrations, H 2 S induces reductive stress by inhibiting the electron transfer chain, causing a reductive shift in the NAD + /NADH, FAD/FADH 2 , and CoQ/CoQH 2 redox couples. [323]The H 2 S-induced redox changes might fan out from the mitochondrion to other compartments such as cytoplasm.Moreover, higher concentrations of H 2 S can also increase metabolic acid-lactate production and impair the pH regulatory system, leading to cell apoptosis. [324]These findings implicate that producing exogenous high levels of H 2 S may represent a potential selective anti-cancer therapeutic strategy.However, progress in the development of H 2 S therapy is hindered by the limited availability of donors and their poor stability during blood circulation, as well as their lack of specificity for targeting tumors. [325]To achieve precisely controllable H 2 S therapy, various H 2 S-generating nanosystems, including those delivering H 2 S donors such as PSD, [325a,b] SH-ASA, [325c] and anethole dithiothione group [325d] (Table 3).For example, Li et al. have developed a polysulfide-based theranostics approach for detecting and treating TNBC using cyanines as a ratiometric PA probe [325a] (Figure 10a).Polysulfide within the nanosystem quickly released H 2 S to a therapeutic concentration, which is real-time sensed by cyanines based on ratiometric PA signals (PA 707 /PA 808 ).This polysulfide-based theranostics exhibited real-time H 2 S monitoring and tumor pinpointing.In addition, the unique characteristics of tumor cells and external stimuli have also been utilized for the localized generation of H 2 S. [326] For instance, Cen et al. have synthesized ZnS@BSA nanoclusters via a selfassembly approach for enhanced immunotherapy.Upon acidic TME, zinc and sulfur ions were released from the nanoclusters.The released zinc ions improved the catalytic enzyme activity of cGAS and significantly enhanced cGAS/STING signals, while sulfur ions react with intracellular hydrogen ions to generate intracellular H 2 S that specifically inhibit the activity of catalase and thus lead to the accumulation of ROS in tumor cells. [327]326e] As shown in Figure 10b, the magnetic nanoliposome delivery system preferentially targets the tumor tissue when spatiotemporally navigated by an external magnetic field, and the released anethole ditholethione continuously generated large numbers of microsized H 2 S bubbles via an enzymatic trigger.Such a nano-to microsize change enables anethole ditholethione to be localized within the TME, microbubble-enhanced US imaging, and spatiotemporal-bombed combination tumor accurate therapy.326c] In a weakly acidic tumor environment, the NPs can release S 2− and Fe 2+ .The released S 2− combines with H + to in situ produce H 2 S gas, which suppresses the activity of the enzyme cytochrome c oxidase in cancer cells.The PT effect further facilitates the Fe 2+ -mediated Fenton reaction to generate abundant •OH radicals for tumor ablation (Figure 10c).Generation of H 2 Se: A balance of selenium (Se) that forms part of the selenoaminoacids, selenomethionine, and selenocysteine is needed for several biological functions in the human body, and an excess and/or insufficient intake can result in adverse health effects. [328]In addition, Se and Se-containing compounds have been proven to be selectively absorbed by cancer cells and have been shown to function as antioxidant and prooxidant agents, providing the basis for their potential applications in cancer prevention and treatment. [329]The mechanisms of action of Se and Se-containing compounds are very diverse, including GPX-like activity, reduction of oxidative stress, cytotoxic activity, and the triggering of apoptotic events, etc.Several studies have demonstrated that some Se-containing compounds such as aryl diselenides induce cytotoxicity in cancer cells by causing reductive stress rather than oxidative stress. [330]H 2 Se, a highly reactive molecule with reducing properties, is a shared metabolite of dietary Se-compounds and formed from Na 2 SeO 3 via selenodiglutathione through reduction by thiols and NADPH-dependent reductases and released from selenocysteine by lyase action. [331]nder hypoxic conditions, H 2 Se accumulation can lead to reductive stress. [332]However, the half-life for the oxidation of H 2 Se to elemental Se in air-saturated water at pH 7 is quite short (≈30 s [333] ), making the controllable production of H 2 Se technically challenging.Although recently several new small-molecule H 2 Se donors have been developed via bond hydrolysis or by the ─SH moiety of Cys to achieve continuous release of the unstable biomolecule under physiologically relevant conditions, [334] they still suffer from the relatively slow H 2 Se release, poor longcirculation ability and potential toxicity caused by non-specific biodistribution.To address these concerns, Lu's group [335] has developed polyethylene glycol-modified ferrous selenide nanoflowers with NIR-II photoactivatable H 2 Se generation ability, which disintegrated and released H 2 Se upon the irradiation of 1064 nm laser (Figure 10d).This PTT-reduction gas synergistic therapy effectively inhibited liver and lung metastasis in the 4T1 mouse model, offering a promising strategy for the design of H 2 Senanomaterials that trigger reduction stress.
337c,338] Various H 2 carriers including Pd-based nanocrystals, [339] MgB 2 nanosheets, [340] ammonia borane-loaded mesoporous silica NPs, [341] Fe NPs, [342] and CaH 2 [343] were fabricated to achieve the storage, targeted delivery and controlled release of H 2 (Table 3).They are roughly divided into two strategies: 1) Nanocarrier-mediated H 2 delivery: For instance, He et al. have synthesized a 2D MgB 2 nanosheet as a new type of acid-responsive hydrogen-releasing prodrug, realizing hydrogenochemotherapy by the combination of facile oral administration of polyvinylpyrrolidone-encapsulating MgB 2 nanosheet pills with routine intravenous injection of Dox. [340]2) In situ production of H 2 by nanogenerators in response to exogenous or endogenous stimuli: He's group [344] has designed a cubic PdH 0.2 nanocrystal and used it for exogenous H 2 release through NIR light.As shown in Figure 8e, the synthesized PdH 0.2 nanocrystals have exhibited high intratumoral accumulation capability, and their hydrogenation results in the significant enhancement of NIR absorption, which enables the PAI-guided release of bioreductive hydrogen as well as PTT.339a] Liu et al. have designed a micro-galvanic cell by in situ reduction of a small amount of Pt on the surface of Mg rods (Figure 10e).The obtained Mg-based galvanic cell can be etched by water to allow the continuous generation of H 2 and Mg(OH) 2 .After implanting MgG rods into tumors, the continuous generation of H 2 could induce mitochondrial dysfunction and disruption of intracellular redox homeostasis, while the byproduct Mg(OH) 2 could neutralize the acidic TME. [345]Both strategies can significantly enhance the targeted accumulation of H 2 and accelerate therapeutic effects.
Cellular Compartments-Targeted Reductive Damage: The redox potential is different within various intracellular subcellular organelles.For example, mitochondria require a more reduced potential and the reported range of GSH/GSSG is from 20:1 to 40:1; while the endoplasmic reticulum (ER) needs a more oxidative environment for the folding of secretory proteins through the formation of disulfide bridges and the ratio of GSH/GSSG varies from 1:1 to 3:1. [346]Therefore, the ER is susceptible to reducing damage caused by excessive electron pressure, which will disrupt the protein folding state within the ER by breaking disulfide bonds in proteins in an oxidative damage-independent 2021, Wiley-VCH.d) Tumor accumulation of photoreleased H 2 Se from FeSe 2 -PEG nanoflowers induces cell death and inhibits liver and lung metastasis.Reproduced with permission. [335]Copyright 2021, Elsevier.e) The preparation of MgG for TME modulation and enhanced exogenous hydrogen therapy.Reproduced with permission. [345]Copyright 2022, Springer Nature.f) The light-triggered electrons by UCSNKNPs attack protein disulfide bonds, leading to structural disruption of ER, thereby enabling reductive damage cancer therapy.Reproduced with permission. [348]Copyright 2022, Elsevier.g) HANP/GKT831 reduces ROS generation by inhibiting NOX1 and NOX4, resulting in reductive stress for the treatment of PDX tumors.Reproduced with permission. [294]Copyright 2022, American Chemical Society.h) Representation of the in situ reaction of Cu 2 O and endogenous H 2 S triggered PA imaging and PTT for colon cancer.Reproduced with permission. [359]Copyright 2018, Wiley-VCH.
manner. [347]For this purpose, Bu's group [348] has designed a NIRresponsive, ER-targeted electron donor by using Yb 3+ and Tm 3+ co-doped UCNPs as the light converters and oxygen vacancies of self-doped SnO 2−x NPs shell as sacrificial electron donors.As shown in Figure 10f, the generated ultraviolet fluorescence from the UCNPs under NIR irradiation excites the separation of electron-hole pairs in the SnO 2−x shell.Photogenerated electrons abort the protein folding process in the ER via a reductive manner, achieving upregulation of the transcription factor Nrf-2, elevated reductive equivalents, and effective cancer therapeutic outcomes.Notably, this strategy is by the utility of reductive damage by harnessing, rather than antagonizing, the intrinsic antioxidant defenses of cancer cells.This unique electronic interference therapy may have broad indications for other intractable diseases.
In summary, with the advent of new technologies that measure reductive stress and regulate reductive stress, emerging evidence suggests that an overly reductive environment also results in profound cellular damage and dysfunction.According to the abovementioned research data, responsive nanomaterials that can deliver or generate reducing equivalents could disrupt the redox balance in cancer cells and ultimately induce the apoptotic signaling pathway, suggesting that manipulating reductive stress may represent a new therapeutic avenue for cancers defined by specific genotypes.However, responsive nanomaterials targeting reductive stress are rare, and this strategy is expected to receive extensive attention and research.Moreover, further study is needed to fully understand the role of reductive stress induced by nanomaterials on cell metabolism and its impact on protein function and the potential compartmentalization, which would help translate this strategy into new cancer therapies.

Responsive Nanosystems Suppressing Reductive Stress to Indirectly Promote Oxidative Damage
As described earlier, cancer cells produce excessive levels of antioxidants and high concentrations of reduced nucleotide cofactors to overcome oxidative damage.Therefore, inhibiting intracellular antioxidant systems (e.g., antioxidant molecules (GSH and NADPH), reducing cofactors (NADH and FADH), and antioxidant enzymes (HO-1) is a potential strategy to enhance the efficiency of ROS-mediated cancer therapy.
NADPH is well known as an essential electron donor and an indispensable cofactor that is used for transferring and reserving reduction potential for numerous anabolic reactions. [349]A growing body of evidence has shown that with the metabolic reprogramming of NADPH, cancer cells increase the demand for NADPH for power redox defense and anabolic reactions to sustain their rapid growth. [350]It has been reported that selectively lowering levels of NAD(H) and NADPH by targeting NAD(P)H-synthesis pathways or NAD(P)H-related enzymes impairs tumor metabolism and induces excessive ROS accumulation, leading to cell toxicity and death. [351]293c] Recently, Yang's group [294] has reported the preparation of HANP nanoparticles by self-assembly and encapsulation of the NOX1/4 dual inhibitor GKT831 (HANP/GKT831) (Figure 10g).Compared to GKT831 alone, HANP-encapsulated GKT831 showed robust inhibition of ROS production and cell growth in the human cancer cell lines and the breast cancer patient-derived xenograft models.On the contrary, recent studies have demonstrated that excess accumulation of NADH leads to disruption of de novo lipid, amino acid, and nucleotide biosynthesis due to the decreased electron acceptors. [352]Recently, Zhang et al. reported a selective H 2 O 2 electrochemical nanosensor, which is prepared by electrodeposition of Prussian blue (PB) and polyethylenedioxythiophene (PEDOT) onto carbon fiber nanoelectrode.With this nanosensor, they find that the level of intracellular H 2 O 2 increases with NADH treatment, and intratumoral injection of high-dose of NADH can inhibit tumor growth in mice, providing a potential therapeutic strategy for cancer therapy. [353]Proposing the concept of hybrid bacteria, Sun et al. conjugated Dox onto the surface of a non-pathogenic E. coli strain overexpressing glucose dehydrogenase through a stable linker of amide bonds.More amounts of NADPH are produced in tumor sites by glucose dehydrogenase within E. coli, promoting the generation of toxic ROS within the tumor.The toxic ROS generation further amplifies the therapeutic effect of DOX-mediated chemotherapy and immunotherapy. [298]n addition to NAD(P)H, GSH is also overproduced in tumor cells (≈1-10 mm, 4 times greater than healthy cells) to counteract the comparatively raised H 2 O 2 level, [354] providing tumor cells with robust resistance to ROS-mediated treatments.355a,b,356] To deplete GSH, Tan et al. have utilized the MnO 2 nanosheet system to enhance the chlorin e6-mediated PDT effect.In this system, MnO 2 nanosheets reduced the cellular levels of GSH. [357]Wang et al. synthesized a nanosized copperbased MOF through one-step self-assembly between copper ions and the catechol ligand, which could oxidize GSH into GSSG and simultaneously disassemble due to the reduction of coordinated copper.Subsequently, the released catechol ligands and Cu + promoted the production of intracellular ROS. [358]Yang's group [359] creatively discovered that the strong binding ability of cuprous oxide (Cu 2 O) to endogenous H 2 S in colon tumor sites is beneficial for tumor elimination (Figure 10h).After entering the colon tumor site, Cu 2 O reacts with endogenous H 2 S to form copper sulfide, which exhibits NIR absorption and can be used for PA imaging and PTT.For reducing GSH synthesis, Chen et al. reported salicylazosulfapyridine (SASP)/ZnO nanoparticles (NPs) to enhance the PDT effect of ZnO NPs by impairing GSH synthesis via SASP-induced inhibition of the cystine/glutamate antiporter. [360]n addition, since the cellular antioxidant defense mechanism is highly complex, simply decreasing the level of a single antioxidant molecule may have a limited impact on improving the efficiency of ROS-mediated cancer therapy.Zhong et al. developed a versatile PDT nanosensitizer containing GSH inhibitor l-buthionine sulfoximine and the HO-1 inhibitor protoporphyrin zinc(II) to suppress the innate antioxidant defense system of cancer cells in a two-pronged manner. [361]The Trx/TrxRs system, [362] which mitigates ROS stress through thiol and selenol groups, is another pivotal system.Cys is crucial for maintaining the TXN oxidation-reduction systems. [363]Based on this, Zhang's group [364] has constructed a nanoplatform FePt@SiO 2 NPs (sSFP).After the nanoplatform reached the tumor site, FePt initiated the Fenton reaction, causing an increase in intracellular free radicals (Ferroptosis).Furthermore, siRNA on the material interfered with xCT expression, blocking Cys uptake, and depleting intracellular Cys.Cys depletion disrupted the oxidationreduction system, amplifying the ferroptosis effect on the tumor.Detection of GSH and TXN proteins in cells and tumor tissues showed that the nanoplatform simultaneously disrupted both the GSH oxidation-reduction system and the TXN oxidationreduction system.In a separate study, Cai et al. [365] loaded Se nanoparticles onto natural -glucan nanotubes to prepare BFP-Se nanomaterials.Metabolomics revealed that BFP-Se-related metabolic reactions primarily pertained to oxidative stress in liver cancer cells.Similarly, Western blot experiments demonstrated that BFP-Se-depleted intracellular GSH, inhibited the expression of Txnip/Trx and Nrf-2/GPX4-related antioxidant systems, ultimately inducing apoptosis and ferroptosis in liver cells.These studies offer valuable insights into the design of nanomaterials capable of simultaneously disrupting the oxidation-reduction balance from the source.

Responsive Nanosystems for Boosting ROS/RNS Production and Inhibiting Antioxidant Mechanisms
As discussed in Part 3.1, the ROS/RNS-dependent strategies can cause tumor cell death, and regulate immunogenicity through ICD for immunotherapy.However, considering intricate redox homeostasis in the TME and the heightened ROS elimination due to elevated antioxidant mechanisms, the ROS/RNS-dependent strategies are still restricted by low ROS production efficiency, the short lifespan of ROS, and insufficient immune activation.The challenges lie in simultaneously reinforcing ROS/RNS production and reducing their expenditure via modulating antioxidant mechanisms.To address these challenges, some intelligent nanosystems have been developed to produce sufficient ROS/RNS, combined with the depletion of accumulated reducing substances (such as GSH) or the inhibition of over-expressed antioxidant enzymes like catalase.GSH, as the major cellular antioxidant, plays an important role in maintaining the balance between oxidation and antioxidation.The GSH concentration in cancer cells can reach as high as 10 mm, at least fourfold that in normal cells, which scavenge excessive ROS.Depleting intracellular GSH breaks the redox homeostasis and causes ROS accumulation, thus boosting the antitumor efficiency of ROS/RNS-based cancer therapies.Therefore, to address these challenges, various functional nanosystems combining GSH-consuming ability and ROS/RNSproducing ability have been proposed to magnify oxidative stress in cancer cells and improve the outcomes of ROS/RNS-based therapy [356,366] (Table 4) These systems can be divided into two strategies: 1) Consuming existing GSH within cancer cells by oxidizing GSH or directly reacting with the thiol group of GSH.For example, some inorganic nanomaterials such as platinum (Pt), [333] copper (Cu), [367] gold (Au), [368] manganese dioxide (MnO 2 ), [369] etc., and some organic molecule-based nanosystem such as 5-norbornene-2-carbaldehyde, [370] hemin [371] etc., are capable of interacting with GSH and effectively deplete intracellular GSH in cancer cells.Fan et al. designed an intelligent nanopar-ticle constructed by synthesizing tetrasulfide bond-doped mesoporous silica nanoparticles, followed by conjugating GOx on the surface and loading l-Arg into the mesopores.This nanoparticle possesses GSH depletion, H 2 O 2 self-supply, and l-Arg loading properties for multimodal cancer therapy. [348]2) Inhibiting GSH biosynthesis from the upstream pathway.Huang et al. construct a homotypic cancer cell membrane-camouflaged iron-small interfering RNA nanohybrid.The SLC7A11-targeted siRNA in the nanohybrid inhibits the biosynthesis of GSH by cutting off the supply of intracellular Cys, resulting in the inactivation of GPX4 and the accumulation of ROS generated from the Fenton reaction induced by iron.Both effects further increase the accretion of lipid peroxides to enhance iron-induced ferroptosis. [372]esides GSH, increased expression of antioxidant enzymes in cells like catalase also decreases ROS production, [362,373] limiting the effect of ROS/RNS-based cancer therapies. [374]Previous investigations have revealed that exogenous H 2 S can inhibit catalase activity in cancer cells. [375]Inspired by this biochemical event, several nanosystems that integrate the H 2 S donor with the ROS generator can generate H 2 S gas and ROS in response to the acidic TME, [376] achieving a "1+1>2" synergistic antitumor treatment (Table 5).Besides, He et al. developed Z-scheme SnS 1.68 -WO 2.41 nanocatalyst to achieve NIR-photocatalytic generation of oxidative holes, which oxidizes/consumes intratumoral over-expressed GSH and simultaneously generate hydrogen molecules.The generated hydrogen molecules and GSH consumption inhibited cellular energy metabolism and destroyed the cellular anti-oxidation defense system by GSH decrease and subsequent ROS increase, realizing combined hole/hydrogen therapy. [377]Similarly, Du's group also designed an asymmetric and lollipop-like nanostructure consisting of gold nanorod/titanium dioxide to produce ROS and H 2 in TME for enhanced PTT/PDT against hypoxic tumors. [378]bove in vitro and in vivo results manifested that the integration of ROS/RNS production with the inhibition of antioxidant mechanisms could overcome the limitation of the ROS/RNSdependent strategies and holds great promise to acquire the synergistic enhanced therapeutic effect.Future studies are needed to understand the antioxidant mechanisms of different types of cancer cells and design intelligent nanosystems that can precisely control the generation of ROS/RNS-based strategy and the suppression of specific antioxidant mechanisms.

Conclusion and Future Prospective
In general, intracellular ROS levels are higher in cancer cells than in normal counterparts.Cancer cells protect themselves from oxidative stress through the upregulation of multiple antioxidant pathways to buffer ROS levels for survival and growth, causing greater tumor survival and chemo/radioresistance.Disrupting the redox balance in cancer through overloading cancer cells or its environment with excessive oxidative stress or modulating the reductive stress underlying the regulation of antioxidant enzymes and metabolic pathways in the TME will provide novel anticancer therapeutic approaches.By taking advantage of responsive nanomaterials, a series of responsive nanomaterials have been designed to target the redox imbalance in cancer cells for selectively destroying cancer cells, triggering mechanisms such as apoptosis, autophagy, necrosis, or ferroptosis.These agents could either •OH multivalent V (CDT, SDT) multivalent V -multivalent V elements simultaneously initiate CDT (low-valence V) and GSH depletion (high-valence V), combining the SDT effect of V to triple oxidative stress [388]   H@Gd-NCPs •OH Gd (Radiotherapy) Gd in H@Gd-NCPs sensitizes X-rays to generate ROS, while Hemin enhances peroxidase-like properties to deplete GSH.This oxidative stress process induces ICD, combined with immune checkpoint blockade therapy to achieve primary, distant, and metastatic tumor treatment [389]   Au-TiO   conversion causes GSH consumption to initiate ferroptosis [395]   LDNPs@Fe/Mn-ZIF-8   For colon cancer treatment, nanomaterials exert therapeutic effect by depleting H 2 S. Abbreviation: ADS, Anti-oxidation defense system; GNRs, Gold nanorods; NPMOF: Nano-porphyrin metal-organic frameworks.
activate a direct increase in ROS production and/or induce an indirect elevation of ROS by inhibiting or decreasing the antioxidative processes of ROS removal above the safe threshold or cause the accumulation of superfluous reducing equivalents leading to the death of cancer cells.Therefore, in this review, we introduced stimuli-responsive nanomaterials that selectively tip the redox imbalance in cancer cells over the threshold for activating cell death in response to external stimuli and tumor internal stimuli.While significant progress has been made in nanomaterials targeting redox imbalance, ongoing research is needed to address the remaining limitations and difficulties for their development into effective anti-tumor treatments.
First, a large number of responsive nanomaterials are being studied for their ability to regulate the redox balance in cancer cells for the management of cancer.It remains to be seen which of these will prove to be the most clinically effective agents.To develop them to become an actual anti-tumor treatment, it is important to more thoroughly understand the transition of the metabolic profile under nanomaterials-induced oxidative and reductive damages in the TME.A recent study [50b] employed mRNA analysis to identify differential gene expression profiles associated with cell apoptosis and ferroptosis in various breast cancer cell lines.Metere et al. also have reported that oxidative stress provokes the alteration of the metabolic profile in thyroid cancer cells.Through metabolomic analysis, this study revealed increases in lactate and aromatic amino acids, such as tyrosine and phenylalanine, and an average decrease in citric acid in thyroid cancer tissue compared to healthy tissue. [380]Therefore, the application of genomic/transcriptomic/proteomic analysis and metabolomic analysis might help understand the antitumor mechanism of redox-regulating nanomaterials.
Second, most tumor cells exhibit abnormally high expression of ROS due to factors such as hypoxia. [381]However, certain tumor cells such as human breast cancer cells and cancer stem cells were proposed not to maintain high ROS levels as they have increased antioxidant capacity. [382]This enables them to acquire tolerance/resistance to chemotherapy and radiotherapy.Therefore, it is crucial to understand the redox states of tumor cells as well as the oxidative and reductive stress underlying the regulation of antioxidant enzymes and metabolic pathways in different types of tumors, which will provide a guide on designing nanomaterials specific to different tumors.
Third, the redox homeostasis of tumors is not a static balance, and it is constantly subjected to dynamic biochemical changes and influenced by the heterogeneity of the TME, the stroma, endothelium, immune cells, and circulating cancer cells. [26]For example, a large number of evidence indicates progressively increased ROS production with advanced stages of malignancy.Early-onset primary tumors will more closely resemble normal cells with greater antioxidative defense systems and be less likely to succumb to the actions of excessive ROS production.Responsive nanomaterials that can activate a direct increase in ROS production and/or indirect elevation of ROS will have a greater impact on preventing metastasis and addressing more advanced stages of cancer.After undergoing metabolic reprogramming to higher antioxidative defense mechanisms, cancer cells might be more susceptible to the cytotoxic actions of ROS-based therapy.Moreover, ROS-based responsive nanomaterials might be used in combination with commonly used chemotherapeutic agents and other forms of cancer treatment, such as radiotherapy.Therefore, it is important to monitor the transition of redox homeostasis and optimize the timing of administration for the clinical application of responsive nanomaterials.Incorporating redox state-imaging agents into redox-regulating nanomaterials might allow for spatial and temporal monitoring of the transition of the redox homeostasis of tumors during the treatment process, which will help to understand the subcellular localization of nanomaterials and their pathway or mechanism to exert redox regulation.
Finally, although enormous progress has been made in using nanomaterials for inducing excessive reactive oxidative species generation to induce tumor destruction, only a few nanomedicines have been approved in clinics or clinical trials.Fortunately, several nanomaterials exhibit vast clinical implications and exciting therapeutic potential. [379]However, no actively targeted or stimulus-responsive nanomedicine has yet been granted regulatory approval.The complexity of the nanobio interface in the dynamic, highly heterogeneous pathological characteristics of malignant tumors, poor controllability and low specificity of nanosystems, and difficulty of large-scale and repeated preparation are still insurmountable challenges for the transition of responsive nanomaterials from bench to bedside.The application of artificial intelligence and machine learning may improve the simulation and modeling process of in vitro and in vivo pharmacokinetics and pharmacodynamics, absorption, distribution, metabolism, and excretion of nanomaterials, enhancing the understanding of their specific toxicity mechanisms, thus accelerating the development of safe and effective nanomedicine.

Figure 1 .
Figure 1.Schematic illustration of abnormal imbalance in the redox systems of tumor cells.

Figure 2 .
Figure 2. Classification of the nanomaterials that induce redox imbalance.

Figure 3 .
Figure 3.The category of stimuli-responsive nanomaterials induces the generation of excessive reactive oxidative species.
etc.For example, Lin et al. have designed a MnO 2 -encapsulated and

Figure 5 .
Figure 5. Schematic illustration of nanomaterials induced RNS the generation of RNS under internal stimuli for cancer treatment.a)The approach of stimuli-responsive nanomaterials to induce RNS generation.b) Nanoassemblies of S-nitrosylation polymers as a NO nanogenerator enable controlled NO release in response to tumoral GSH, inducing immunogenic cell death (ICD) to enhance immunotherapy.Reproduced with permission.[146]Copyright 2022, American Chemical Society.c) Schematic illustration of a nanozyme-based NO generator, CeO 2 -AT to specifically produce NO under the catalysis of NOS in mitochondria of cancer cells for selective oncotherapy.Reproduced with permission.[147]Copyright 2022, Springer Nature.d) A supramolecular Pt/NO prodrug nanoassemblies strategy realized ONOO − -potentiated chemotherapy of liver cancer.Reproduced with permission.[148]Copyright 2021, American Chemical Society.

Figure 6 .
Figure 6.Schematic illustration of nanomaterials induced the generation of other radicals under internal stimuli for cancer treatment.a)The mechanisms for acid-triggered sequential release of •OH and CO by the FeCO-MnO 2 @MSN nanomedicine for synergetic therapy.Reproduced with permission.[175c]Copyright 2019, Springer Nature.b) AIPH/MSN-TPP@Lipo/DTX-FA NPs released alkyl radicals in mitochondria at high temperatures and then combined with DTX to enhance the cell apoptosis process.Reproduced with permission.[176]Copyright 2020, Elsevier.c) PNSO releases sulfate radicals under TME and triggers ROS storms to lead to immunotherapy through the ICD process.Reproduced with permission.[177]Copyright 2020, American Chemical Society.Abbreviations: TPP, 5,10,15,20-Tetraphenylporphyrin.

Figure 7 .
Figure 7. Schematic illustration of nanomaterials generated reactive oxidative species under light-stimuli for cancer treatment.a) NIR-I light upconverted to blue light to irradiate Blebbistatin to generate •OH through oxygen-independent PDT.Reproduced with permission.[196]Copyright 2021, Wiley-VCH.b) Schematic illustration of the acidic TME-responsive ECPM nanohybrids for NIR-induced PDT and downconverted NIR-II imaging.Reproduced with permission.[200]Copyright 2020, American Chemical Society.c) Schematic illustration of DPP-NF NPs for controllable "on-off" release of NO under light/dark conditions.Reproduced with permission.[211]Copyright 2018, Wiley-VCH.d) Design of a controlled CO delivery nanomaterial for improving cancer therapy.Reproduced with permission.[214]Copyright 2020, American Chemical Society.e) Schematic illustration showing the controlled release and generation of alkyl radicals upon irradiation by NIR laser.Reproduced with permission.[217]Copyright 2017, Wiley-VCH.f) CNPPtCP/si(c-fos) generates oxygen-independent N 3 • for endo/lysosomal escape, accompanied with the subsequently released Pt(II) and si(c-fos) for synergistic cancer therapy.Reproduced with permission.[228]Copyright 2020, American Chemical Society.g) BSO@Fe 3 O 4 -Sch nanocomposites produce •SO 4 − under the attacking of abundant •OH, which is produced through Fenton reaction trigged by NIR laser.Reproduced with permission.[229]Copyright 2021, Wiley-VCH.h) Schematic diagram of UCSAP synthesis and •Cl generation for hypoxic tumor therapy.Reproduced with permission.[231]Copyright 2020, Wiley-VCH.
Light activation of peroxodisulfate and peroxymonosulfatebased nanosystems could generate •SO 4 − , irrespective of the amount of H 2 O 2 , O 2 , and pH value.As an avant-garde paradigm, Wu et al. constructed a multifunctional hybrid nanoplatform with a hollow Fe 3 O 4 core and cell pseudopod-like Schwertmannite shell.The Fe 3 O 4 core conducts Fenton reaction catalysts to produce •OH, while Schwertmannite shells convert SO 4 2− into •SO 4

Figure 8 .
Figure 8. Schematic illustration of nanomaterials generated reactive oxidative species under external stimuli for cancer treatment.a) Schematic illustration of the radiation-induced production of ROS by PMAO-KI NPs.Reproduced with permission.[246]Copyright 2021, American Chemical Society.b) Schematic illustration showing the synthetic route to RBS-T-SCNPs and the X-ray-controlled ONOO − generation for on-demand cancer radiotherapy.Reproduced with permission.[248]Copyright 2018, Wiley-VCH.c) Schematic illustration of the electrophilic ligands released by ZIF-82-PVP generate 2 O 2 and H 2 O to produce •OH• and •O 2 − , respectively.Then, the produced OH• subsequently triggers the decomposition of CF 3 SO 2 Na into CF 3 •.The generation of •OH•, •O 2 − and CF 3 • could reverse the cisplatin resistance and intensify ovarian cancer treatment by activating apoptosis and ferroptosis programs (Figure 8e).Moreover, the integration of the •SO 4 − generation agent, PMS, into a Pd-catalyzed hydrogenated mesoporous titanium dioxide exhibited excellent US-excited ROS generation under the support of Pd nanozyme-mediated O 2 supply and concurrently •SO 4

Figure 9 .
Figure 9. Schematic illustration of nanomaterials regulating endogenous oxidation substances.a) The release of OH-CPT from the cleavage of TBSOCPT by fluoride ions activates NOX to upregulate intracellular H 2 O 2 and amplify the iron-induced Fenton reaction to achieve synergistic therapy.Reproduced with permission.[295]Copyright 2022, American Chemical Society.b) Upper figure: MAO oxidase inhibitory ability of different concentrations of MAOI-like AuNC, bottom figure: Computational analysis of MAOI activity of size-dependent AuNCs.Reproduced with permission.[305]Copyright 2021, Elsevier.c) AGuIX nanoparticles amplify the ferroptosis system by inhibiting the Nrf-2-GSH-GPX4 signaling pathway.Reproduced with permission.[312]Copyright 2022, Springer Nature.

Figure 10 .
Figure 10.Schematic illustration of nanomaterials inducing reduction imbalances.a) Ratiometric photoacoustic (PA) monitored TME-initiated H 2 S therapy for TNBC.Reproduced with permission. [325a] Copyright 2020, Wiley-VCH.b) Nano to the micro conversion of AMLs for US/MR dual-modal imaging and the spatiotemporal-bombed combination tumor accurate therapy.Reproduced with permission. [326e] Copyright 2017, American Chemical Society.c) H 2 S released from Fe 1−x S-PVP under acidic TME inhibits COX IV activity to control tumor growth.Reproduced with permission. [326c] Copyright

2
-y to achieve controlled hydrogen evolution and hole-synchronized GSH oxidation, thereby disrupting cellular ADS to synergistically induce tumor apoptosis porphyrin on NPMOF under PDT do not interfere with each other and synergistically induce cell by GNRs under NIR were injected into the conduction band of L-TiO 2 through the Schottky junction.Both of these electrons and the remaining holes on GNRs generate •OH, •O

Xumeng
Wu is a Ph.D. candidate at Harbin Institute of Technology and majored in Biomedical Engineering.He received his Master's degree from the University of South China in 2020.His research focuses on how nanomaterials regulate cancer cell's microenvironment, especially in redox aspects.Ziqi Zhou is a Ph.D. candidate at Harbin Institute of Technology and majored in Biomedical Engineering.She received her Bachelor's and Master's degrees at Dalian Maritime University and Harbin Institute of Technology, respectively.Her research interests are bioenergy nanosystems and related photoelectric conversion processes.Kai Li received his Bachelor's and Master's degrees in Biology from the Harbin Institute of Technology, then he pursued his Ph.D. research in Immunology and Cancer in the Professor Akinori Takaoka group at the Institute for Genetic Medicine, Hokkaido University in Japan.He returned to Harbin Institute of Technology to do postdoctoral research after completing his Ph.D. degree in 2016, and now he is an associate professor at this institute.His work primarily focuses on understanding the mechanisms and developing therapeutic strategies to combat chemotherapy resistance in pancreatic cancer, including investigating the impact of epigenetic regulation and tumor microenvironment on chemotherapy resistance in pancreatic cancer, as well as exploring nanomedicine-based treatments for pancreatic cancer.Shaoqin Liu received her Bachelor's degree and Ph.D. degree from Wuhan University of Hydraulic and Electric Engineering in 1994 and from Changchun Institute of Applied Chemistry (Chinese Academy of Science) in 1999, respectively.She started her Chemistry research career under Professor Shaojun Dong's group at Changchun Institute of Applied Chemistry to develop polyoxometalates-based thin film.After her Ph.D. degree, she moved to the Max-Planck Institute of Colloids and Interface as a Humboldt Fellow.She developed polyoxometalates-based functional materials.In 2004, she joined the National Research Council of Canada as an NESRC fellow to study direct methanol fuel cells.In 2007, she started her academic career as a Full Professor at Harbin Institute of Technology.She was sponsored by the National Science Fund for Outstanding Young Scholars in 2018.Her current research interests include the preparation of nanostructured materials and their applications in energy, biosensing, and cancer therapy.She is an Associate Editor for the Journal of Materials Chemistry B and Materials Advance.

Table 1 .
Comparison of different reactive oxidative species.

Table 2 .
Summary of the representative nanomaterials that generate ROS under internal stimuli.

Table 3 .
Summary of the representative nanomaterials that generate reduction substances.

Table 4 .
Summary of the representative nanomaterials that simultaneously generate free radicals/RNS with GSH depletion.

Table 5 .
Summary of the representative nanomaterials that simultaneously generate ROS and generate H Under acidic TEM, the four-sulfide bond-breaking process consumes GSH, and at the same time, the released CPO generates 1 S while producing NIR and PA signal, which not only cause mitochondrial damage, but the decomposition product of ZM1068-ketone can generate 1 S to inhibit catalase activity and induce iron pools in cells to accumulate •OH.